feat: add smooth theme transitions

Apply resolved themes immediately beneath a 500 ms snapshot cross-fade shared by visible top-level windows. Keep interruption, resize, shutdown, and reduced-animation paths deterministic, with real Qt lifecycle coverage.

Signed-off-by: Loren Eteval <loren.eteval@proton.me>

tests/test_theme_transition.py

@@ -0,0 +1,199 @@
+# Copyright (C) 2024–present  Loren Eteval & contributors <loren.eteval@proton.me>
+#
+# This file is part of Furious.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+"""Verify real Qt animation and ownership for application theme transitions."""
+
+from __future__ import annotations
+
+import unittest
+
+from PySide6 import QtCore
+from PySide6.QtTest import QSignalSpy
+from PySide6.QtWidgets import QWidget
+
+from shiboken6 import isValid
+
+from Furious.Qt import ThemeTransition
+
+from tests.support import application, processQtEvents, waitFor
+
+
+class ThemeTransitionTest(unittest.TestCase):
+    """Exercise cross-fades through the real Qt event loop."""
+
+    def setUp(self):
+        """Create per-test windows while retaining one process-wide application."""
+        application()
+
+        self.windows = []
+        self.transitions = []
+
+    def tearDown(self):
+        """Release every transient overlay, animation, coordinator, and window."""
+        for transition in self.transitions:
+            transition.stop()
+            transition.deleteLater()
+
+        for window in self.windows:
+            window.close()
+            window.deleteLater()
+
+        processQtEvents()
+
+    def createWindow(self):
+        """Create and show one deterministic top-level transition target."""
+        window = QWidget()
+        window.resize(320, 180)
+        window.show()
+
+        self.windows.append(window)
+
+        processQtEvents()
+
+        return window
+
+    def createTransition(self, windows, *, duration=60, enabled=True):
+        """Create one coordinator with deterministic animation policy."""
+        transition = ThemeTransition(
+            duration=duration,
+            windowProvider=lambda: tuple(windows),
+            animationsEnabled=lambda: enabled,
+        )
+
+        self.transitions.append(transition)
+
+        return transition
+
+    @staticmethod
+    def overlays(window):
+        """Return the transition overlays currently owned by *window*."""
+        return window.findChildren(
+            QWidget,
+            ThemeTransition.OverlayObjectName,
+            QtCore.Qt.FindChildOption.FindDirectChildrenOnly,
+        )
+
+    def testThemeIsAppliedImmediatelyThenSnapshotCompletesAndIsRemoved(self):
+        """Keep destination state live beneath one real fading snapshot."""
+        window = self.createWindow()
+        transition = self.createTransition([window])
+        started = QSignalSpy(transition.transitionStarted)
+        finished = QSignalSpy(transition.transitionFinished)
+
+        def applyDarkTheme():
+            """Represent the application's synchronous destination-theme commit."""
+            window.setProperty('testTheme', 'Dark')
+            window.setStyleSheet('background-color: #10151d;')
+
+        transition.apply(applyDarkTheme)
+
+        self.assertEqual(window.property('testTheme'), 'Dark')
+        self.assertTrue(transition.isRunning())
+        self.assertEqual(transition.activeOverlayCount(), 1)
+        self.assertEqual(len(self.overlays(window)), 1)
+        self.assertEqual(started.count(), 1)
+        self.assertTrue(
+            transition.findChildren(QtCore.QPropertyAnimation),
+            'the transition must use a real QPropertyAnimation',
+        )
+
+        self.assertTrue(waitFor(lambda: not transition.isRunning()))
+        processQtEvents()
+
+        self.assertEqual(finished.count(), 1)
+        self.assertEqual(transition.activeOverlayCount(), 0)
+        self.assertEqual(self.overlays(window), [])
+        self.assertEqual(
+            transition.findChildren(QtCore.QPropertyAnimation),
+            [],
+        )
+
+    def testRapidRepeatedSwitchReplacesRatherThanStacksTransitions(self):
+        """Restart from the visible composite and retain only the newest target."""
+        window = self.createWindow()
+        transition = self.createTransition([window], duration=160)
+        started = QSignalSpy(transition.transitionStarted)
+        finished = QSignalSpy(transition.transitionFinished)
+
+        transition.apply(lambda: window.setProperty('testTheme', 'Dark'))
+        firstOverlay = self.overlays(window)[0]
+
+        transition.apply(lambda: window.setProperty('testTheme', 'Light'))
+
+        self.assertEqual(window.property('testTheme'), 'Light')
+        self.assertEqual(transition.activeOverlayCount(), 1)
+        self.assertEqual(started.count(), 2)
+        self.assertEqual(finished.count(), 1)
+
+        processQtEvents()
+
+        self.assertFalse(isValid(firstOverlay))
+        self.assertEqual(len(self.overlays(window)), 1)
+        self.assertTrue(waitFor(lambda: not transition.isRunning()))
+        processQtEvents()
+
+        self.assertEqual(finished.count(), 2)
+        self.assertEqual(self.overlays(window), [])
+
+    def testMultipleWindowsTransitionAndResizeIndependently(self):
+        """Cancel only a resized window's stale snapshot while peers continue."""
+        firstWindow = self.createWindow()
+        secondWindow = self.createWindow()
+        transition = self.createTransition(
+            [firstWindow, secondWindow],
+            duration=120,
+        )
+        finished = QSignalSpy(transition.transitionFinished)
+
+        def applyLightTheme():
+            """Commit the same destination theme to both top-level windows."""
+            firstWindow.setProperty('testTheme', 'Light')
+            secondWindow.setProperty('testTheme', 'Light')
+
+        transition.apply(applyLightTheme)
+
+        self.assertEqual(transition.activeOverlayCount(), 2)
+        firstWindow.resize(360, 200)
+        processQtEvents()
+
+        self.assertEqual(firstWindow.property('testTheme'), 'Light')
+        self.assertEqual(secondWindow.property('testTheme'), 'Light')
+        self.assertEqual(self.overlays(firstWindow), [])
+        self.assertEqual(transition.activeOverlayCount(), 1)
+        self.assertTrue(transition.isRunning())
+        self.assertTrue(waitFor(lambda: not transition.isRunning()))
+        processQtEvents()
+
+        self.assertEqual(finished.count(), 1)
+        self.assertEqual(self.overlays(secondWindow), [])
+
+    def testDisabledAnimationsNeverDelayOrOverlayTheDestinationTheme(self):
+        """Honor the animation policy while preserving immediate theme activation."""
+        window = self.createWindow()
+        transition = self.createTransition([window], enabled=False)
+        started = QSignalSpy(transition.transitionStarted)
+
+        transition.apply(lambda: window.setProperty('testTheme', 'Dark'))
+
+        self.assertEqual(window.property('testTheme'), 'Dark')
+        self.assertFalse(transition.isRunning())
+        self.assertEqual(started.count(), 0)
+        self.assertEqual(self.overlays(window), [])
+
+
+if __name__ == '__main__':
+    unittest.main()