feat(core): allow back in multishare setup

### core/.changelog.d/7823.changed
@@ -0,0 +1 @@
+Allow going back and changing previous selections during multishare backup creation, and remove the option to interrupt the flow from the checklist and number input screens.

### core/embed/rust/src/ui/api/firmware_micropython.rs
@@ -828,10 +828,11 @@ extern "C" fn new_show_checklist(n_args: usize, args: *const Obj, kwargs: *mut M
         let button: TString = kwargs.get(Qstr::MP_QSTR_button)?.try_into()?;
         let active: usize = kwargs.get(Qstr::MP_QSTR_active)?.try_into()?;
         let items: Obj = kwargs.get(Qstr::MP_QSTR_items)?;
+        let back_button: bool = kwargs.get_or(Qstr::MP_QSTR_back_button, false)?;
 
         let items: [TString<'static>; MAX_CHECKLIST_ITEMS] = util::iter_into_array(items)?;
 
-        let layout = ModelUI::show_checklist(title, button, active, items)?;
+        let layout = ModelUI::show_checklist(title, button, active, items, back_button)?;
         Ok(LayoutObj::new_root(layout)?.into())
     };
     unsafe { util::try_with_args_and_kwargs(n_args, args, kwargs, block) }
@@ -1874,6 +1875,7 @@ pub static mp_module_trezorui_api: Module = obj_module! {
     ///     items: Iterable[str],
     ///     active: int,
     ///     button: str,
+    ///     back_button: bool = False,
     /// ) -> LayoutContext[UiResult]:
     ///     """Checklist of backup steps. Active index is highlighted, previous items have check
     ///     mark next to them. Limited to 3 items."""

### core/embed/rust/src/ui/layout_bolt/ui_firmware.rs
@@ -819,6 +819,7 @@ impl FirmwareUI for UIBolt {
         button: TString<'static>,
         active: usize,
         items: [TString<'static>; MAX_CHECKLIST_ITEMS],
+        _back_button: bool,
     ) -> Result<impl LayoutMaybeTrace, Error> {
         let mut paragraphs = ParagraphVecLong::new();
         for (i, item) in items.into_iter().enumerate() {

### core/embed/rust/src/ui/layout_caesar/ui_firmware.rs
@@ -1004,6 +1004,7 @@ impl FirmwareUI for UICaesar {
         button: TString<'static>,
         active: usize,
         items: [TString<'static>; MAX_CHECKLIST_ITEMS],
+        _back_button: bool,
     ) -> Result<impl LayoutMaybeTrace, Error> {
         let mut paragraphs = ParagraphVecLong::new();
         for (i, item) in items.into_iter().enumerate() {

### core/embed/rust/src/ui/layout_delizia/ui_firmware.rs
@@ -804,6 +804,7 @@ impl FirmwareUI for UIDelizia {
         _button: TString<'static>,
         active: usize,
         items: [TString<'static>; MAX_CHECKLIST_ITEMS],
+        _back_button: bool,
     ) -> Result<impl LayoutMaybeTrace, Error> {
         let mut paragraphs = ParagraphVecLong::new();
         for (i, item) in items.into_iter().enumerate() {

### core/embed/rust/src/ui/layout_eckhart/component_msg_obj.rs
@@ -8,15 +8,15 @@ use super::firmware::{
     TextScreen, TextScreenMsg, ValueInput, ValueInputScreen, ValueInputScreenMsg,
 };
 use crate::micropython::{Error, Obj};
-use crate::ui::component::MsgMap;
+use crate::ui::component::text::paragraphs::{Checklist, ParagraphVecShort};
 #[cfg(not(feature = "clippy"))]
 use crate::ui::component::{
     text::paragraphs::{ParagraphSource, Paragraphs},
     Component, Timeout,
 };
-use crate::ui::flow::FlowMsg;
+use crate::ui::component::{FlowMsg, MsgMap};
 use crate::ui::layout::obj::ComponentMsgObj;
-use crate::ui::layout::result::{CANCELLED, CONFIRMED, INFO};
+use crate::ui::layout::result::{BACK, CANCELLED, CONFIRMED, INFO};
 
 impl ComponentMsgObj for PinKeyboard<'_> {
     fn msg_try_into_obj(&self, msg: Self::Msg) -> Result<Obj, Error> {
@@ -136,6 +136,19 @@ impl<T: ValueInput> ComponentMsgObj for ValueInputScreen<T> {
     }
 }
 
+/// Layout returned by `show_checklist`: the checklist of backup steps with an
+/// optional back button (chevron-up) in place of the cancel button.
+pub type ChecklistScreen =
+    MsgMap<TextScreen<Checklist<ParagraphVecShort<'static>>>, fn(TextScreenMsg) -> Option<FlowMsg>>;
+
+impl ComponentMsgObj for ChecklistScreen {
+    fn msg_try_into_obj(&self, msg: Self::Msg) -> Result<Obj, Error> {
+        // `TryFrom<FlowMsg> for Obj` covers all the emitted variants,
+        // including `FlowMsg::Back`.
+        msg.try_into()
+    }
+}
+
 /// Layout returned by `request_number`: the number input screen with the
 /// "more info" menu button in the header.
 pub type RequestNumberScreen =
@@ -148,6 +161,7 @@ impl ComponentMsgObj for RequestNumberScreen {
         let value: u32 = self.inner().value();
         match msg {
             FlowMsg::Confirmed => Ok((CONFIRMED.as_obj(), value).try_into()?),
+            FlowMsg::Back => Ok((BACK.as_obj(), value).try_into()?),
             FlowMsg::Info => Ok((INFO.as_obj(), value).try_into()?),
             msg => msg.try_into(),
         }

### core/embed/rust/src/ui/layout_eckhart/ui_firmware.rs
@@ -800,8 +800,12 @@ impl FirmwareUI for UIEckhart {
         // the header emits `FlowMsg::Info` and the layout returns the
         // currently displayed number along with the result (see
         // `ComponentMsgObj for RequestNumberScreen`).
+        // The left action bar button (chevron-up) emits
+        // `ValueInputScreenMsg::Cancelled`; interpret it as going back. Going
+        // back is the only way out of the prompt -- interrupting the flow is
+        // not possible.
         let map_fn: fn(ValueInputScreenMsg) -> Option<FlowMsg> = |msg| match msg {
-            ValueInputScreenMsg::Cancelled => Some(FlowMsg::Cancelled),
+            ValueInputScreenMsg::Cancelled => Some(FlowMsg::Back),
             ValueInputScreenMsg::Confirmed(_) => Some(FlowMsg::Confirmed),
             ValueInputScreenMsg::Menu => Some(FlowMsg::Info),
             ValueInputScreenMsg::Changed(_) => None,
@@ -812,9 +816,10 @@ impl FirmwareUI for UIEckhart {
                 description.unwrap_or(TString::empty()),
             )
             .with_header(Header::new(title).with_menu_button())
-            .with_action_bar(ActionBar::new_single(Button::with_text(
-                TR::buttons__confirm.into(),
-            )))
+            .with_action_bar(ActionBar::new_double(
+                Button::with_icon(theme::ICON_CHEVRON_UP),
+                Button::with_text(TR::buttons__confirm.into()),
+            ))
             .map(map_fn),
         );
         Ok(layout)
@@ -979,6 +984,7 @@ impl FirmwareUI for UIEckhart {
         button: TString<'static>,
         active: usize,
         items: [TString<'static>; MAX_CHECKLIST_ITEMS],
+        back_button: bool,
     ) -> Result<impl LayoutMaybeTrace, Error> {
         let mut paragraphs = ParagraphVecShort::new();
         for (i, item) in items.into_iter().enumerate() {
@@ -1003,13 +1009,30 @@ impl FirmwareUI for UIEckhart {
         .with_done_offset(theme::CHECKLIST_DONE_OFFSET)
         .with_current_offset(theme::CHECKLIST_CURRENT_OFFSET);
 
+        let action_bar = if back_button {
+            ActionBar::new_double(
+                Button::with_icon(theme::ICON_CHEVRON_UP),
+                Button::with_text(button),
+            )
+        } else {
+            ActionBar::new_single(Button::with_text(button))
+        };
+
+        // The left action bar button (only present when `back_button` is set)
+        // emits `TextScreenMsg::Cancelled`; interpret it as going back. Going
+        // back is the only way out of the checklist -- interrupting the flow
+        // is not possible.
+        let map_fn: fn(TextScreenMsg) -> Option<FlowMsg> = |msg| match msg {
+            TextScreenMsg::Cancelled => Some(FlowMsg::Back),
+            TextScreenMsg::Menu => Some(FlowMsg::Info),
+            TextScreenMsg::Confirmed => Some(FlowMsg::Confirmed),
+        };
+
         let layout = RootComponent::new(
             TextScreen::new(checklist_content)
                 .with_header(Header::new(title))
-                .with_action_bar(ActionBar::new_double(
-                    Button::with_icon(theme::ICON_CROSS),
-                    Button::with_text(button),
-                )),
+                .with_action_bar(action_bar)
+                .map(map_fn),
         );
 
         Ok(layout)

### core/embed/rust/src/ui/ui_firmware.rs
@@ -420,6 +420,7 @@ pub trait FirmwareUI {
         button: TString<'static>,
         active: usize,
         items: [TString<'static>; MAX_CHECKLIST_ITEMS],
+        back_button: bool,
     ) -> Result<impl LayoutMaybeTrace, Error>;
 
     fn show_danger(

### core/mocks/generated/trezorui_api.pyi
@@ -594,6 +594,7 @@ def show_checklist(
     items: Iterable[str],
     active: int,
     button: str,
+    back_button: bool = False,
 ) -> LayoutContext[UiResult]:
     """Checklist of backup steps. Active index is highlighted, previous items have check
     mark next to them. Limited to 3 items."""

### core/src/apps/management/reset_device/__init__.py
@@ -3,6 +3,7 @@
 
 import storage
 import storage.device as storage_device
+import trezorui_api
 from trezor import TR
 from trezor.crypto import hmac, slip39
 from trezor.enums import BackupType, MessageType
@@ -224,14 +225,65 @@ async def _backup_slip39_basic(
 
     await handler.intro()
 
-    # get number of shares
-    await layout.slip39_show_checklist(0, advanced=False)
-    share_count = await layout.slip39_prompt_number_of_shares(num_of_words)
+    share_count: int | None = None
+    share_threshold: int | None = None
 
-    # get threshold
-    await layout.slip39_show_checklist(1, advanced=False, count=share_count)
-    share_threshold = await layout.slip39_prompt_threshold(share_count)
+    # Let the user go back and forth between the steps, keeping the already
+    # entered values. `BACK` results can only occur on layouts that support
+    # going back; other layouts only ever confirm and march forward.
+    step = 0
+    while True:
+        if step == 0:
+            # checklist: set number of shares
+            # (no going back from the first step)
+            await layout.slip39_show_checklist(0, advanced=False)
+            step = 1
+        elif step == 1:
+            # get number of shares
+            result = await layout.slip39_prompt_number_of_shares(
+                num_of_words, init_value=share_count
+            )
+            if result is trezorui_api.BACK:
+                step = 0
+            else:
+                assert isinstance(result, int)
+                share_count = result
+                step = 2
+        elif step == 2:
+            # checklist: set threshold
+            assert share_count is not None
+            result = await layout.slip39_show_checklist(
+                1, advanced=False, count=share_count, back_button=True
+            )
+            step = 1 if result is trezorui_api.BACK else 3
+        elif step == 3:
+            # get threshold
+            assert share_count is not None
+            result = await layout.slip39_prompt_threshold(
+                share_count, init_value=share_threshold
+            )
+            if result is trezorui_api.BACK:
+                step = 2
+            else:
+                assert isinstance(result, int)
+                share_threshold = result
+                step = 4
+        else:
+            # checklist: write down and check the shares
+            assert share_count is not None and share_threshold is not None
+            result = await layout.slip39_show_checklist(
+                2,
+                advanced=False,
+                count=share_count,
+                threshold=share_threshold,
+                back_button=True,
+            )
+            if result is trezorui_api.BACK:
+                step = 3
+            else:
+                break
 
+    assert share_count is not None and share_threshold is not None
     mnemonics = _get_slip39_mnemonics(
         encrypted_master_secret,
         group_threshold,
@@ -240,12 +292,77 @@ async def _backup_slip39_basic(
     )
 
     # show and confirm individual shares
-    await layout.slip39_show_checklist(
-        2, advanced=False, count=share_count, threshold=share_threshold
-    )
     await layout.slip39_basic_show_and_confirm_shares(handler, mnemonics[0])
 
 
+async def _prompt_slip39_groups(
+    num_of_words: int,
+    groups_count: int,
+    groups: list[tuple[int, int]],
+) -> bool:
+    """Prompt for the share count and threshold of each group, letting the
+    user go back and forth between the groups. The entered (threshold, share
+    count) pairs are stored in `groups`, preselecting the previously entered
+    values.
+
+    Returns False if the user wants to go back from the first group,
+    otherwise True.
+    """
+    group_index = 0
+    # share count of the group currently being configured
+    pending_share_count: int | None = None
+    # whether to prompt for the group's share count or go straight to its
+    # threshold (used when going back to a previous group)
+    prompt_shares = True
+    while group_index < groups_count:
+        if prompt_shares:
+            # get number of shares of the current group
+            result = await layout.slip39_prompt_number_of_shares(
+                num_of_words,
+                group_index,
+                init_value=(
+                    pending_share_count
+                    if pending_share_count is not None
+                    else groups[group_index][1]
+                    if group_index < len(groups)
+                    else None
+                ),
+            )
+            if result is trezorui_api.BACK:
+                if group_index == 0:
+                    return False  # back to the checklist
+                # back to the previous group's threshold
+                group_index -= 1
+                pending_share_count = groups[group_index][1]
+                prompt_shares = False
+                continue
+            assert isinstance(result, int)
+            pending_share_count = result
+
+        # get threshold of the current group
+        assert pending_share_count is not None
+        result = await layout.slip39_prompt_threshold(
+            pending_share_count,
+            group_index,
+            init_value=(groups[group_index][0] if group_index < len(groups) else None),
+        )
+        if result is trezorui_api.BACK:
+            # back to the current group's number of shares
+            prompt_shares = True
+            continue
+        assert isinstance(result, int)
+        group = (result, pending_share_count)
+        if group_index < len(groups):
+            groups[group_index] = group
+        else:
+            assert group_index == len(groups)
+            groups.append(group)
+        pending_share_count = None
+        group_index += 1
+        prompt_shares = True
+    return True
+
+
 async def _backup_slip39_advanced(
     handler: layout.BackupHandler,
     encrypted_master_secret: bytes,
@@ -254,24 +371,71 @@ async def _backup_slip39_advanced(
 ) -> None:
     await handler.intro()
 
-    # get number of groups
-    await layout.slip39_show_checklist(0, advanced=True)
-    groups_count = await layout.slip39_advanced_prompt_number_of_groups()
+    groups_count: int | None = None
+    group_threshold: int | None = None
+    groups: list[tuple[int, int]] = []
 
-    # get group threshold
-    await layout.slip39_show_checklist(1, advanced=True, count=groups_count)
-    group_threshold = await layout.slip39_advanced_prompt_group_threshold(groups_count)
-
-    # get shares and thresholds
-    await layout.slip39_show_checklist(
-        2, advanced=True, count=groups_count, threshold=group_threshold
-    )
-    groups = []
-    for i in range(groups_count):
-        share_count = await layout.slip39_prompt_number_of_shares(num_of_words, i)
-        share_threshold = await layout.slip39_prompt_threshold(share_count, i)
-        groups.append((share_threshold, share_count))
+    # Let the user go back and forth between the steps, keeping the already
+    # entered values. `BACK` results can only occur on layouts that support
+    # going back; other layouts only ever confirm and march forward.
+    step = 0
+    while True:
+        if step == 0:
+            # checklist: set number of groups
+            # (no going back from the first step)
+            await layout.slip39_show_checklist(0, advanced=True)
+            step = 1
+        elif step == 1:
+            # get number of groups
+            result = await layout.slip39_advanced_prompt_number_of_groups(
+                init_value=groups_count
+            )
+            if result is trezorui_api.BACK:
+                step = 0
+            else:
+                assert isinstance(result, int)
+                groups_count = result
+                # keep the already entered groups that still fit
+                del groups[groups_count:]
+                step = 2
+        elif step == 2:
+            # checklist: set group threshold
+            assert groups_count is not None
+            result = await layout.slip39_show_checklist(
+                1, advanced=True, count=groups_count, back_button=True
+            )
+            step = 1 if result is trezorui_api.BACK else 3
+        elif step == 3:
+            # get group threshold
+            assert groups_count is not None
+            result = await layout.slip39_advanced_prompt_group_threshold(
+                groups_count, init_value=group_threshold
+            )
+            if result is trezorui_api.BACK:
+                step = 2
+            else:
+                assert isinstance(result, int)
+                group_threshold = result
+                step = 4
+        elif step == 4:
+            # checklist: set sizes and thresholds of the individual groups
+            assert groups_count is not None and group_threshold is not None
+            result = await layout.slip39_show_checklist(
+                2,
+                advanced=True,
+                count=groups_count,
+                threshold=group_threshold,
+                back_button=True,
+            )
+            step = 3 if result is trezorui_api.BACK else 5
+        else:
+            # get sizes and thresholds of the individual groups
+            assert step == 5 and groups_count is not None
+            if await _prompt_slip39_groups(num_of_words, groups_count, groups):
+                break
+            step = 4  # back to the checklist
 
+    assert group_threshold is not None
     mnemonics = _get_slip39_mnemonics(
         encrypted_master_secret, group_threshold, groups, extendable
     )

### core/src/trezor/ui/layouts/bolt/reset.py
@@ -77,7 +77,8 @@ async def slip39_show_checklist(
     advanced: bool,
     count: int | None = None,
     threshold: int | None = None,
-) -> None:
+    back_button: bool = False,
+) -> trezorui_api.UiResult:
     items = (
         (
             TR.reset__slip39_checklist_set_num_shares,
@@ -98,9 +99,7 @@ async def slip39_show_checklist(
         active=step,
         items=items,
     ) as layout:
-        return await raise_if_not_confirmed(
-            layout, "slip39_checklist", ButtonRequestType.ResetDevice
-        )
+        return await interact(layout, "slip39_checklist", ButtonRequestType.ResetDevice)
 
 
 async def _prompt_number(
@@ -147,13 +146,17 @@ async def _prompt_number(
 
 
 def slip39_prompt_threshold(
-    num_of_shares: int, group_id: int | None = None
-) -> Awaitable[int]:
-    count = num_of_shares // 2 + 1
+    num_of_shares: int, group_id: int | None = None, init_value: int | None = None
+) -> Awaitable[int | trezorui_api.UiResult]:
     # min value of share threshold is 2 unless the number of shares is 1
     # number of shares 1 is possible in advanced slip39
     min_count = min(2, num_of_shares)
     max_count = num_of_shares
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = num_of_shares // 2 + 1
 
     def description(count: int) -> str:
         if group_id is None:
@@ -203,11 +206,15 @@ def info(count: int) -> str:
 
 
 def slip39_prompt_number_of_shares(
-    num_words: int, group_id: int | None = None
-) -> Awaitable[int]:
-    count = 5
+    num_words: int, group_id: int | None = None, init_value: int | None = None
+) -> Awaitable[int | trezorui_api.UiResult]:
     min_count = 1
     max_count = 16
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = 5
 
     def description(i: int) -> str:
         if group_id is None:
@@ -238,10 +245,16 @@ def description(i: int) -> str:
     )
 
 
-def slip39_advanced_prompt_number_of_groups() -> Awaitable[int]:
-    count = 5
+def slip39_advanced_prompt_number_of_groups(
+    init_value: int | None = None,
+) -> Awaitable[int | trezorui_api.UiResult]:
     min_count = 2
     max_count = 16
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = 5
     description = TR.reset__group_description
     info = TR.reset__group_info
 
@@ -256,10 +269,16 @@ def slip39_advanced_prompt_number_of_groups() -> Awaitable[int]:
     )
 
 
-def slip39_advanced_prompt_group_threshold(num_of_groups: int) -> Awaitable[int]:
-    count = num_of_groups // 2 + 1
+def slip39_advanced_prompt_group_threshold(
+    num_of_groups: int, init_value: int | None = None
+) -> Awaitable[int | trezorui_api.UiResult]:
     min_count = 1
     max_count = num_of_groups
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = num_of_groups // 2 + 1
     description = TR.reset__required_number_of_groups
     info = TR.reset__advanced_group_threshold_info
 

### core/src/trezor/ui/layouts/caesar/reset.py
@@ -4,7 +4,7 @@
 from trezor import TR
 from trezor.enums import ButtonRequestType
 
-from ..common import interact, raise_if_not_confirmed
+from ..common import interact
 from . import confirm_action, show_success, show_warning
 
 CONFIRMED = trezorui_api.CONFIRMED  # global_import_cache
@@ -104,7 +104,8 @@ async def slip39_show_checklist(
     advanced: bool,
     count: int | None = None,
     threshold: int | None = None,
-) -> None:
+    back_button: bool = False,
+) -> trezorui_api.UiResult:
     items = (
         (
             TR.reset__slip39_checklist_num_shares,
@@ -125,9 +126,7 @@ async def slip39_show_checklist(
         active=step,
         items=items,
     ) as layout:
-        return await raise_if_not_confirmed(
-            layout, "slip39_checklist", ButtonRequestType.ResetDevice
-        )
+        return await interact(layout, "slip39_checklist", ButtonRequestType.ResetDevice)
 
 
 async def _prompt_number(
@@ -161,8 +160,8 @@ async def _prompt_number(
 
 
 async def slip39_prompt_threshold(
-    num_of_shares: int, group_id: int | None = None
-) -> int:
+    num_of_shares: int, group_id: int | None = None, init_value: int | None = None
+) -> int | trezorui_api.UiResult:
     await confirm_action(
         "slip39_prompt_threshold",
         TR.words__title_threshold,
@@ -171,11 +170,15 @@ async def slip39_prompt_threshold(
         verb_cancel=None,
     )
 
-    count = num_of_shares // 2 + 1
     # min value of share threshold is 2 unless the number of shares is 1
     # number of shares 1 is possible in advanced slip39
     min_count = min(2, num_of_shares)
     max_count = num_of_shares
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = num_of_shares // 2 + 1
 
     if group_id is not None:
         title = f"{TR.words__title_threshold} - {TR.words__title_group} {group_id + 1}"
@@ -192,8 +195,8 @@ async def slip39_prompt_threshold(
 
 
 async def slip39_prompt_number_of_shares(
-    _num_words: int, group_id: int | None = None
-) -> int:
+    _num_words: int, group_id: int | None = None, init_value: int | None = None
+) -> int | trezorui_api.UiResult:
     await confirm_action(
         "slip39_shares",
         TR.reset__title_number_of_shares,
@@ -202,9 +205,13 @@ async def slip39_prompt_number_of_shares(
         verb_cancel=None,
     )
 
-    count = 5
     min_count = 1
     max_count = 16
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = 5
 
     if group_id is not None:
         title = f"# {TR.words__title_shares} - {TR.words__title_group} {group_id + 1}"
@@ -220,10 +227,16 @@ async def slip39_prompt_number_of_shares(
     )
 
 
-def slip39_advanced_prompt_number_of_groups() -> Awaitable[int]:
-    count = 5
+def slip39_advanced_prompt_number_of_groups(
+    init_value: int | None = None,
+) -> Awaitable[int | trezorui_api.UiResult]:
     min_count = 2
     max_count = 16
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = 5
 
     return _prompt_number(
         TR.reset__title_number_of_groups,
@@ -234,10 +247,16 @@ def slip39_advanced_prompt_number_of_groups() -> Awaitable[int]:
     )
 
 
-def slip39_advanced_prompt_group_threshold(num_of_groups: int) -> Awaitable[int]:
-    count = num_of_groups // 2 + 1
+def slip39_advanced_prompt_group_threshold(
+    num_of_groups: int, init_value: int | None = None
+) -> Awaitable[int | trezorui_api.UiResult]:
     min_count = 1
     max_count = num_of_groups
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = num_of_groups // 2 + 1
 
     return _prompt_number(
         TR.reset__title_group_threshold,

### core/src/trezor/ui/layouts/delizia/reset.py
@@ -94,7 +94,8 @@ async def slip39_show_checklist(
     advanced: bool,
     count: int | None = None,
     threshold: int | None = None,
-) -> None:
+    back_button: bool = False,
+) -> ui.UiResult:
     items = _slip_39_checklist_items(step, advanced, count, threshold)
     with trezorui_api.show_checklist(
         title=TR.reset__title_shamir_backup,
@@ -107,6 +108,7 @@ async def slip39_show_checklist(
         )
     if result != CONFIRMED:
         raise ActionCancelled
+    return result
 
 
 def _slip_39_checklist_items(
@@ -199,13 +201,17 @@ async def _prompt_number(
 
 
 def slip39_prompt_threshold(
-    num_of_shares: int, group_id: int | None = None
-) -> Awaitable[int]:
-    count = num_of_shares // 2 + 1
+    num_of_shares: int, group_id: int | None = None, init_value: int | None = None
+) -> Awaitable[int | ui.UiResult]:
     # min value of share threshold is 2 unless the number of shares is 1
     # number of shares 1 is possible in advanced slip39
     min_count = min(2, num_of_shares)
     max_count = num_of_shares
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = num_of_shares // 2 + 1
 
     description = (
         TR.reset__select_threshold
@@ -234,11 +240,15 @@ def info(count: int) -> str:
 
 
 async def slip39_prompt_number_of_shares(
-    num_words: int, group_id: int | None = None
-) -> int:
-    count = 5
+    num_words: int, group_id: int | None = None, init_value: int | None = None
+) -> int | ui.UiResult:
     min_count = 1
     max_count = 16
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = 5
 
     description = (
         TR.reset__num_of_shares_how_many
@@ -264,10 +274,16 @@ async def slip39_prompt_number_of_shares(
     )
 
 
-async def slip39_advanced_prompt_number_of_groups() -> int:
-    count = 5
+async def slip39_advanced_prompt_number_of_groups(
+    init_value: int | None = None,
+) -> int | ui.UiResult:
     min_count = 2
     max_count = 16
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = 5
     description = TR.reset__group_description
     info = TR.reset__group_info
 
@@ -282,10 +298,16 @@ async def slip39_advanced_prompt_number_of_groups() -> int:
     )
 
 
-async def slip39_advanced_prompt_group_threshold(num_of_groups: int) -> int:
-    count = num_of_groups // 2 + 1
+async def slip39_advanced_prompt_group_threshold(
+    num_of_groups: int, init_value: int | None = None
+) -> int | ui.UiResult:
     min_count = 1
     max_count = num_of_groups
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = num_of_groups // 2 + 1
     description = TR.reset__required_number_of_groups
     info = TR.reset__advanced_group_threshold_info
 

### core/src/trezor/ui/layouts/eckhart/reset.py
@@ -6,13 +6,11 @@
 from trezor.wire import ActionCancelled
 
 from ..common import interact
-from . import raise_if_not_confirmed, show_success
+from . import BACK, CANCELLED, CONFIRMED, raise_if_not_confirmed, show_success
 
 if TYPE_CHECKING:
     from collections.abc import Awaitable, Callable, Sequence
 
-CONFIRMED = trezorui_api.CONFIRMED  # global_import_cache
-
 
 async def show_share_words(
     share_words: Sequence[str],
@@ -118,19 +116,26 @@ async def slip39_show_checklist(
     advanced: bool,
     count: int | None = None,
     threshold: int | None = None,
-) -> None:
+    back_button: bool = False,
+) -> ui.UiResult:
     items = _slip_39_checklist_items(step, advanced, count, threshold)
     with trezorui_api.show_checklist(
         title=TR.reset__title_shamir_backup,
         button=TR.buttons__continue,
         active=step,
         items=items,
+        back_button=back_button,
     ) as layout:
         result = await interact(
-            layout, "slip39_checklist", ButtonRequestType.ResetDevice
+            layout,
+            "slip39_checklist",
+            ButtonRequestType.ResetDevice,
+            raise_on_cancel=None,
         )
-    if result != CONFIRMED:
+    if result is CANCELLED:
+        # not reachable from the UI, only via debuglink
         raise ActionCancelled
+    return result
 
 
 def _slip_39_checklist_items(
@@ -175,7 +180,7 @@ async def _prompt_number(
     min_count: int,
     max_count: int,
     br_name: str,
-) -> int:
+) -> int | ui.UiResult:
     from trezor.ui.layouts.menu import Menu, leaf_from_layout, show_menu
 
     with trezorui_api.request_number(
@@ -195,8 +200,9 @@ async def _prompt_number(
             )
             br_name_once = None  # ButtonRequest should be sent only once
 
-            if result is trezorui_api.CANCELLED:
-                raise ActionCancelled  # user cancelled request number prompt
+            if result is CANCELLED:
+                # not reachable from the UI, only via debuglink
+                raise ActionCancelled
 
             if __debug__ and not isinstance(result, tuple):
                 # sent by debuglink. debuglink does not change the number of
@@ -208,6 +214,9 @@ async def _prompt_number(
                 assert isinstance(value, int)
                 return value
 
+            if result is BACK:
+                return BACK
+
             if result is trezorui_api.INFO:
                 # shows the menu with the "more info" screen
                 leaf = leaf_from_layout(
@@ -223,13 +232,17 @@ async def _prompt_number(
 
 
 def slip39_prompt_threshold(
-    num_of_shares: int, group_id: int | None = None
-) -> Awaitable[int]:
-    count = num_of_shares // 2 + 1
+    num_of_shares: int, group_id: int | None = None, init_value: int | None = None
+) -> Awaitable[int | ui.UiResult]:
     # min value of share threshold is 2 unless the number of shares is 1
     # number of shares 1 is possible in advanced slip39
     min_count = min(2, num_of_shares)
     max_count = num_of_shares
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = num_of_shares // 2 + 1
 
     description = (
         TR.reset__select_threshold
@@ -258,11 +271,15 @@ def info(count: int) -> str:
 
 
 async def slip39_prompt_number_of_shares(
-    num_words: int, group_id: int | None = None
-) -> int:
-    count = 5
+    num_words: int, group_id: int | None = None, init_value: int | None = None
+) -> int | ui.UiResult:
     min_count = 1
     max_count = 16
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = 5
 
     description = (
         TR.reset__num_of_shares_how_many
@@ -288,10 +305,16 @@ async def slip39_prompt_number_of_shares(
     )
 
 
-async def slip39_advanced_prompt_number_of_groups() -> int:
-    count = 5
+async def slip39_advanced_prompt_number_of_groups(
+    init_value: int | None = None,
+) -> int | ui.UiResult:
     min_count = 2
     max_count = 16
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = 5
     description = TR.reset__group_description
     info = TR.reset__group_info
 
@@ -306,10 +329,16 @@ async def slip39_advanced_prompt_number_of_groups() -> int:
     )
 
 
-async def slip39_advanced_prompt_group_threshold(num_of_groups: int) -> int:
-    count = num_of_groups // 2 + 1
+async def slip39_advanced_prompt_group_threshold(
+    num_of_groups: int, init_value: int | None = None
+) -> int | ui.UiResult:
     min_count = 1
     max_count = num_of_groups
+    if init_value is not None:
+        # returning to the prompt with a previously entered value
+        count = max(min_count, min(init_value, max_count))
+    else:
+        count = num_of_groups // 2 + 1
     description = TR.reset__required_number_of_groups
     info = TR.reset__advanced_group_threshold_info
 

### tests/device_tests/test_msg_backup_device.py
@@ -36,8 +36,10 @@
     FlowAdapter,
     InputFlowBip39Backup,
     InputFlowSlip39AdvancedBackup,
+    InputFlowSlip39AdvancedBackupBackNavigation,
     InputFlowSlip39AdvancedCustomBackup,
     InputFlowSlip39BasicBackup,
+    InputFlowSlip39BasicBackupBackNavigation,
     InputFlowSlip39CustomBackup,
     normal,
     try_to_cancel,
@@ -188,6 +190,61 @@ def test_backup_slip39_advanced(
     assert expected_ms == actual_ms
 
 
+@pytest.mark.models("eckhart")  # going back is supported only on Eckhart
+@pytest.mark.setup_client(needs_backup=True, mnemonic=MNEMONIC_SLIP39_BASIC_20_3of6)
+def test_backup_slip39_basic_back_navigation(
+    session: Session, backup_method: messages.BackupMethod
+):
+    with session.test_ctx as client:
+        IF = InputFlowSlip39BasicBackupBackNavigation(session, method=backup_method)
+        client.set_input_flow(IF.get())
+        device.backup(session, backup_method=backup_method)
+
+    session.refresh_features()
+    assert session.features.initialized is True
+    assert (
+        session.features.backup_availability == messages.BackupAvailability.NotAvailable
+    )
+    assert session.features.unfinished_backup is False
+    assert session.features.no_backup is False
+    assert session.features.backup_type is messages.BackupType.Slip39_Basic
+
+    # the flow went back and forth and settled on 3-of-3 shares
+    assert len(IF.mnemonics) == 3
+    expected_ms = shamir.combine_mnemonics(MNEMONIC_SLIP39_BASIC_20_3of6)
+    actual_ms = shamir.combine_mnemonics(IF.mnemonics)
+    assert expected_ms == actual_ms
+
+
+@pytest.mark.models("eckhart")  # going back is supported only on Eckhart
+@pytest.mark.setup_client(needs_backup=True, mnemonic=MNEMONIC_SLIP39_ADVANCED_20)
+def test_backup_slip39_advanced_back_navigation(
+    session: Session, backup_method: messages.BackupMethod
+):
+    with session.test_ctx as client:
+        IF = InputFlowSlip39AdvancedBackupBackNavigation(
+            session, method=backup_method
+        )
+        client.set_input_flow(IF.get())
+        device.backup(session, backup_method=backup_method)
+
+    session.refresh_features()
+    assert session.features.initialized is True
+    assert (
+        session.features.backup_availability == messages.BackupAvailability.NotAvailable
+    )
+    assert session.features.unfinished_backup is False
+    assert session.features.no_backup is False
+    assert session.features.backup_type is messages.BackupType.Slip39_Advanced
+
+    # the flow went back and forth and settled on groups 2-of-3 and 3-of-5
+    # with group threshold 2
+    assert [len(group) for group in IF.mnemonics] == [3, 5]
+    expected_ms = shamir.combine_mnemonics(MNEMONIC_SLIP39_ADVANCED_20)
+    actual_ms = shamir.combine_mnemonics(IF.mnemonics[0][:2] + IF.mnemonics[1][:3])
+    assert expected_ms == actual_ms
+
+
 SLIP39_CUSTOM_PARAMS = [
     (threshold, count, adapt_flow)
     for threshold, count in ((1, 1), (2, 2), (3, 5))

### tests/input_flows.py
@@ -2717,6 +2717,187 @@ def input_flow_eckhart(self) -> BRGeneratorType:
         self.debug.press_yes()
 
 
+class InputFlowSlip39BasicBackupBackNavigation(InputFlowBase):
+    """Going back and forth in the SLIP39 basic backup creation, changing the
+    number of shares and the threshold."""
+
+    def __init__(
+        self,
+        client: Client | DebugSession,
+        method: messages.BackupMethod = messages.BackupMethod.Display,
+    ):
+        super().__init__(client)
+        self.mnemonics: list[str] = []
+        self.method = method
+
+    def input_flow_eckhart(self) -> BRGeneratorType:
+        assert self.method in (
+            messages.BackupMethod.Display,
+            messages.BackupMethod.N1W1,
+        )
+        if self.method is messages.BackupMethod.Display:
+            assert (yield).name == "backup_intro"
+            self.debug.press_yes()
+
+        # checklist: set number of shares (no going back from the first step)
+        assert (yield).name == "slip39_checklist"
+        self.debug.press_yes()
+
+        # number of shares prompt: change the default 5 to 3
+        assert (yield).name == "slip39_shares"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 5
+        self.debug.click(self.debug.screen_buttons.number_input_minus())
+        self.debug.click(self.debug.screen_buttons.number_input_minus())
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # checklist: set threshold -- go back from here
+        assert (yield).name == "slip39_checklist"
+        self.debug.click(self.debug.screen_buttons.cancel())
+
+        # number of shares prompt: the previously entered value is preselected
+        assert (yield).name == "slip39_shares"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 3
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # checklist: set threshold
+        assert (yield).name == "slip39_checklist"
+        self.debug.press_yes()
+
+        # threshold prompt: increase the default 2 to 3
+        assert (yield).name == "slip39_threshold"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 2
+        self.debug.click(self.debug.screen_buttons.number_input_plus())
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # checklist: write down the shares -- go back from here
+        assert (yield).name == "slip39_checklist"
+        self.debug.click(self.debug.screen_buttons.cancel())
+
+        # threshold prompt: the previously entered value is preselected
+        assert (yield).name == "slip39_threshold"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 3
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # checklist: write down the shares
+        assert (yield).name == "slip39_checklist"
+        self.debug.press_yes()
+
+        if self.method is messages.BackupMethod.Display:
+            assert (yield).name == "backup_warning"
+            self.debug.press_yes()
+
+        # Mnemonic phrases
+        self.mnemonics = yield from load_N_shares(self.debug, 3, self.method)
+
+
+class InputFlowSlip39AdvancedBackupBackNavigation(InputFlowBase):
+    """Going back and forth in the SLIP39 advanced backup creation, changing
+    the group parameters. Only for Eckhart, which supports going back in this
+    flow."""
+
+    def __init__(
+        self,
+        client: Client | DebugSession,
+        method: messages.BackupMethod = messages.BackupMethod.Display,
+    ):
+        super().__init__(client)
+        self.mnemonics: list[list[str]] = []
+        self.method = method
+
+    def input_flow_eckhart(self) -> BRGeneratorType:
+        assert self.method in (
+            messages.BackupMethod.Display,
+            messages.BackupMethod.N1W1,
+        )
+        if self.method is messages.BackupMethod.Display:
+            assert (yield).name == "backup_intro"
+            self.debug.press_yes()
+
+        # checklist: set number of groups (no going back from the first step)
+        assert (yield).name == "slip39_checklist"
+        self.debug.press_yes()
+
+        # number of groups prompt: change the default 5 to 2
+        assert (yield).name == "slip39_groups"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 5
+        self.debug.click(self.debug.screen_buttons.number_input_minus())
+        self.debug.click(self.debug.screen_buttons.number_input_minus())
+        self.debug.click(self.debug.screen_buttons.number_input_minus())
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # checklist: set group threshold
+        assert (yield).name == "slip39_checklist"
+        self.debug.press_yes()
+
+        # group threshold prompt: keep the default 2
+        assert (yield).name == "slip39_group_threshold"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 2
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # checklist: set sizes and thresholds of the groups
+        assert (yield).name == "slip39_checklist"
+        self.debug.press_yes()
+
+        # group 1 shares prompt: change the default 5 to 3
+        assert (yield).name == "slip39_shares"
+        self.debug.click(self.debug.screen_buttons.number_input_minus())
+        self.debug.click(self.debug.screen_buttons.number_input_minus())
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # group 1 threshold prompt: keep the default 2
+        assert (yield).name == "slip39_threshold"
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # group 2 shares prompt: keep the default 5 -- go back from here
+        assert (yield).name == "slip39_shares"
+        self.debug.click(self.debug.screen_buttons.cancel())
+
+        # group 1 threshold prompt: the previously entered value is preselected
+        assert (yield).name == "slip39_threshold"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 2
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # group 2 shares prompt: keep the default 5
+        assert (yield).name == "slip39_shares"
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # group 2 threshold prompt: change the default 3 to 4 -- go back
+        assert (yield).name == "slip39_threshold"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 3
+        self.debug.click(self.debug.screen_buttons.number_input_plus())
+        self.debug.click(self.debug.screen_buttons.cancel())
+
+        # group 2 shares prompt: the previously entered value is preselected
+        assert (yield).name == "slip39_shares"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 5
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        # group 2 threshold prompt: the default 3 is shown again -- keep it
+        assert (yield).name == "slip39_threshold"
+        layout = self.debug.read_layout()
+        assert layout.find_unique_value_by_key("value", None, only_type=int) == 3
+        self.debug.click(self.debug.screen_buttons.ok())
+
+        if self.method is messages.BackupMethod.Display:
+            assert (yield).name == "backup_warning"
+            self.debug.press_yes()
+
+        # Mnemonic phrases - show & confirm shares for all groups
+        # 2 groups: 2-of-3 and 3-of-5
+        self.mnemonics = yield from load_N_groups(
+            self.debug, [(2, 3), (3, 5)], self.method
+        )
+
+
 class InputFlowSlip39AdvancedResetRecovery(InputFlowBase):
     def __init__(
         self,