Tests: UI: Add UI tests for fullscreen and stacked fullscreen handling

There are a lot of (fragile) cases we can cover with tests here, this is
just a start.

Pull Request: https://projects.blender.org/blender/blender/pulls/151722

tests/python/CMakeLists.txt

@@ -1497,6 +1497,8 @@ if(WITH_UI_TESTS)
     test_workspace.sanity_check_storyboarding
     test_workspace.sanity_check_vfx
     test_workspace.sanity_check_video_editing
+    test_fullscreen.wm_toggle_fullscreen
+    test_fullscreen.wm_toggle_stacked_fullscreen_file_browser
   )
   foreach(ui_test ${_ui_tests})
     add_blender_test_ui(

tests/python/ui_simulate/test_fullscreen.py

@@ -0,0 +1,96 @@
+# SPDX-FileCopyrightText: 2025 Blender Authors
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+"""
+This file does not run anything, its methods are accessed for tests by ``run_blender_setup.py``.
+"""
+
+
+def _test_window(windows_exclude=None):
+    import bpy
+    wm = bpy.data.window_managers[0]
+    if windows_exclude is None:
+        return wm.windows[0]
+    for window in wm.windows:
+        if window not in windows_exclude:
+            return window
+    return None
+
+
+def _test_vars(window):
+    import unittest
+    from modules.easy_keys import EventGenerate
+    return (
+        EventGenerate(window),
+        unittest.TestCase(),
+    )
+
+
+def _call_by_name(e, text: str):
+    yield e.f3()
+    yield e.text(text)
+    yield e.ret()
+
+
+def wm_toggle_fullscreen():
+    e, t = _test_vars(window := _test_window())
+
+    t.assertNotEqual(len(window.screen.areas), 1, "Expected a window with more than one area")
+
+    yield from _call_by_name(e, "Toggle Maximize Area")
+    yield e.ret()
+    t.assertEqual(len(window.screen.areas), 1)
+
+    yield from _call_by_name(e, "Toggle Maximize Area")
+    yield e.ret()
+    t.assertNotEqual(len(window.screen.areas), 1)
+
+
+def wm_toggle_stacked_fullscreen_file_browser():
+    import bpy
+
+    e, t = _test_vars(window := _test_window())
+
+    t.assertNotEqual(len(window.screen.areas), 1, "Expected a window with more than one area")
+
+    # Ensure temporary file browsers will be opened in a maximized screen.
+    bpy.context.preferences.view.filebrowser_display_type = 'SCREEN'
+
+    # Open temporary file browser.
+    yield e.ctrl.o()
+    t.assertEqual(len(window.screen.areas), 1)
+    t.assertEqual(window.screen.areas[0].type, 'FILE_BROWSER')
+
+    # Escape should leave the temporary file browser maximized screen.
+    yield e.esc()
+    t.assertNotEqual(len(window.screen.areas), 1)
+    t.assertNotEqual(window.screen.areas[0].type, 'FILE_BROWSER')
+
+    # Maximize a normal area.
+    yield from _call_by_name(e, "Toggle Maximize Area")
+    yield e.ret()
+    t.assertEqual(len(window.screen.areas), 1)
+    t.assertNotEqual(window.screen.areas[0].type, 'FILE_BROWSER')
+
+    # Open a file browser in a stacked fullscreen.
+    yield e.ctrl.o()
+    t.assertEqual(len(window.screen.areas), 1)
+    t.assertEqual(window.screen.areas[0].type, 'FILE_BROWSER')
+
+    # Pressing 'Escape' should return to the previous full screen.
+    yield e.esc()
+    t.assertEqual(len(window.screen.areas), 1)
+    t.assertNotEqual(window.screen.areas[0].type, 'FILE_BROWSER')
+    restored_area = window.screen.areas[0]
+
+    # Pressing 'Escape' again shouldn't cause any change.
+    yield e.esc()
+    t.assertEqual(len(window.screen.areas), 1)
+    t.assertNotEqual(window.screen.areas[0].type, 'FILE_BROWSER')
+    t.assertEqual(restored_area, window.screen.areas[0])
+
+    # Restore to non-maximized area.
+    yield from _call_by_name(e, "Toggle Maximize Area")
+    yield e.ret()
+    t.assertNotEqual(len(window.screen.areas), 1)