test: add interface_gui.py to test bitcoin-gui startup via RPC

diff --git a/src/qt/guiutil.cpp b/src/qt/guiutil.cpp
index e0966b9c..6e5d1759 100644
--- a/src/qt/guiutil.cpp
+++ b/src/qt/guiutil.cpp
@@ -290,6 +290,8 @@ bool hasEntryData(const QAbstractItemView *view, int column, int role)
 
 void LoadFont(const QString& file_name)
 {
+    // The qminimal plugin does not provide font loading support.
+    if (QApplication::platformName() == "minimal") return;
     const int id = QFontDatabase::addApplicationFont(file_name);
     assert(id != -1);
 }
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index eac3ec59..91a6afb1 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -29,6 +29,7 @@ function(create_test_config)
   set_configure_variable(ENABLE_EXTERNAL_SIGNER ENABLE_EXTERNAL_SIGNER)
   set_configure_variable(WITH_USDT ENABLE_USDT_TRACEPOINTS)
   set_configure_variable(ENABLE_IPC ENABLE_IPC)
+  set_configure_variable(BUILD_GUI BUILD_GUI)
 
   configure_file(config.ini.in config.ini USE_SOURCE_PERMISSIONS @ONLY)
 endfunction()
diff --git a/test/config.ini.in b/test/config.ini.in
index 20fa36b9..45b69ded 100644
--- a/test/config.ini.in
+++ b/test/config.ini.in
@@ -29,3 +29,4 @@ RPCAUTH=@abs_top_srcdir@/share/rpcauth/rpcauth.py
 @ENABLE_EXTERNAL_SIGNER_TRUE@ENABLE_EXTERNAL_SIGNER=true
 @ENABLE_USDT_TRACEPOINTS_TRUE@ENABLE_USDT_TRACEPOINTS=true
 @ENABLE_IPC_TRUE@ENABLE_IPC=true
+@BUILD_GUI_TRUE@BUILD_GUI=true
diff --git a/test/functional/interface_gui.py b/test/functional/interface_gui.py
new file mode 100755
index 00000000..7f05d720
--- /dev/null
+++ b/test/functional/interface_gui.py
@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+# Copyright (c) The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test that bitcoin-gui starts up and can be stopped via RPC."""
+
+import platform
+
+from test_framework.test_framework import (
+    BitcoinTestFramework,
+    SkipTest,
+)
+
+
+class GuiTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+        self.extra_args = [["-server"]]
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_gui()
+        # On Windows, bitcoin.exe exits immediately when launching bitcoin-gui.exe,
+        # causing the test framework's process monitor to see a premature node exit.
+        # On macOS, bitcoin-qt's Cocoa code assumes NSApp is initialized, but the
+        # minimal Qt platform plugin skips that, causing crashes.
+        # Both issues are likely fixable.
+        if platform.system() in ("Windows", "Darwin"):
+            raise SkipTest("bitcoin-gui test not supported on Windows or macOS")
+
+    def setup_nodes(self):
+        self.extra_init = [{"use_gui": True}]
+        super().setup_nodes()
+
+    def run_test(self):
+        self.log.info("Test that bitcoin-gui starts up and can be stopped via RPC")
+        self.stop_node(0)
+
+
+if __name__ == "__main__":
+    GuiTest(__file__).main()
diff --git a/test/functional/test_framework/test_framework.py b/test/functional/test_framework/test_framework.py
index 64dcbfd7..8559a2ae 100755
--- a/test/functional/test_framework/test_framework.py
+++ b/test/functional/test_framework/test_framework.py
@@ -1078,6 +1078,11 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
         if not self.is_ipc_compiled():
             raise SkipTest("ipc has not been compiled.")
 
+    def skip_if_no_gui(self):
+        """Skip the running test if the GUI has not been compiled."""
+        if not self.is_gui_compiled():
+            raise SkipTest("GUI has not been compiled.")
+
     def skip_if_no_previous_releases(self):
         """Skip the running test if previous releases are not available."""
         if not self.has_previous_releases():
@@ -1160,6 +1165,10 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
         """Checks whether ipc was compiled."""
         return self.config["components"].getboolean("ENABLE_IPC")
 
+    def is_gui_compiled(self):
+        """Checks whether the GUI was compiled."""
+        return self.config["components"].getboolean("BUILD_GUI", fallback=False)
+
     def has_blockfile(self, node, filenum: str):
         return (node.blocks_path/ f"blk{filenum}.dat").is_file()
 
diff --git a/test/functional/test_framework/test_node.py b/test/functional/test_framework/test_node.py
index a7c140a5..de96cc60 100755
--- a/test/functional/test_framework/test_node.py
+++ b/test/functional/test_framework/test_node.py
@@ -112,6 +112,7 @@ class TestNode():
         v2transport=False,
         uses_wallet=False,
         ipcbind=False,
+        use_gui=False,
     ):
         self.index = i
         self.datadir_path = datadir_path
@@ -125,6 +126,7 @@ class TestNode():
         self.binaries = binaries
         self.coverage_dir = coverage_dir
         self.cwd = cwd
+        self.use_gui = use_gui
         self.has_explicit_bind = False
         if extra_conf is not None:
             append_config(self.datadir_path, extra_conf)
@@ -138,7 +140,7 @@ class TestNode():
         # Configuration for logging is set as command-line args rather than in the bitcoin.conf file.
         # This means that starting a bitcoind using the temp dir to debug a failed test won't
         # spam debug.log.
-        self.args = self.binaries.node_argv(need_ipc=ipcbind) + [
+        self.args = self.binaries.node_argv(need_ipc=ipcbind, use_gui=use_gui) + [
             f"-datadir={self.datadir_path}",
             "-logtimemicros",
             "-debug",
@@ -278,6 +280,20 @@ class TestNode():
 
         # add environment variable LIBC_FATAL_STDERR_=1 so that libc errors are written to stderr and not the terminal
         subp_env = dict(os.environ, LIBC_FATAL_STDERR_="1")
+        if self.use_gui:
+            subp_env.setdefault("QT_QPA_PLATFORM", "minimal")
+            subp_env.setdefault("LC_ALL", "nl_NL.UTF-8") # Set language to try to trigger translation bugs
+            if sys.platform.startswith("linux") and "XDG_RUNTIME_DIR" not in subp_env:
+                # Qt prints warnings to stderr when XDG_RUNTIME_DIR is unset or has wrong
+                # permissions (e.g. in CI environments without a desktop session), which
+                # would cause tests to fail due to unexpected stderr output. The two
+                # warnings are:
+                #   "QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'"
+                #   "QStandardPaths: wrong permissions on runtime directory /path, 0755 instead of 0700"
+                # Use a dedicated subdirectory with the required 0700 permissions.
+                xdg_runtime_dir = self.datadir_path / "xdg_runtime"
+                xdg_runtime_dir.mkdir(mode=0o700, exist_ok=True)
+                subp_env["XDG_RUNTIME_DIR"] = str(xdg_runtime_dir)
         if env is not None:
             subp_env.update(env)
 
diff --git a/test/functional/test_framework/util.py b/test/functional/test_framework/util.py
index 7d064d68..24b31928 100644
--- a/test/functional/test_framework/util.py
+++ b/test/functional/test_framework/util.py
@@ -305,11 +305,11 @@ class Binaries:
         "Return argv array that should be used to invoke bitcoin-chainstate"
         return self._argv("chainstate", self.paths.bitcoinchainstate)
 
-    def _argv(self, command, bin_path, need_ipc=False):
+    def _argv(self, command, bin_path, *, need_ipc=False, use_gui=False):
         """Return argv array that should be used to invoke the command.
 
-        It either uses the bitcoin wrapper executable (if BITCOIN_CMD is set or
-        need_ipc is True), or the direct binary path (bitcoind, etc). When
+        It either uses the bitcoin wrapper executable (if BITCOIN_CMD, need_ipc,
+        or use_gui are set), or the direct binary path (bitcoind, etc). When
         bin_dir is set (by tests calling binaries from previous releases) it
         always uses the direct path.
 
@@ -319,11 +319,12 @@ class Binaries:
         """
         if self.bin_dir is not None:
             return [os.path.join(self.bin_dir, os.path.basename(bin_path))]
-        elif self.paths.bitcoin_cmd is not None or need_ipc:
-            # If the current test needs IPC functionality, use the bitcoin
-            # wrapper binary and append -m so it calls multiprocess binaries.
+        elif self.paths.bitcoin_cmd is not None or need_ipc or use_gui:
+            # If the current test needs IPC or GUI functionality, use the
+            # bitcoin wrapper binary and add appropriate options.
             bitcoin_cmd = self.paths.bitcoin_cmd or [self.paths.bitcoin_bin]
-            return self.valgrind_cmd + bitcoin_cmd + (["-m"] if need_ipc else []) + [command]
+            subcommand = "gui" if use_gui and command == "node" else command
+            return self.valgrind_cmd + bitcoin_cmd + (["-m"] if need_ipc else []) + [subcommand]
         else:
             return self.valgrind_cmd + [bin_path]
 
diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index 5bfc7d86..1f7416d6 100755
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -361,6 +361,7 @@ BASE_SCRIPTS = [
     'feature_logging.py',
     'interface_ipc.py',
     'interface_ipc_mining.py',
+    'interface_gui.py',
     'feature_anchors.py',
     'mempool_datacarrier.py',
     'feature_coinstatsindex.py',