Add slider-wipe compare view

Before and after share one transform; before is drawn into after's
rectangle so it is never copied at 4x size. Drag the divider to wipe,
drag elsewhere to pan, wheel zooms around the cursor, double-click fits.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/test_upscale_qt.py b/test_upscale_qt.py
index 526dc37..439807c 100644
--- a/test_upscale_qt.py
+++ b/test_upscale_qt.py
@@ -77,6 +77,25 @@ def test_save_over():
     assert not os.path.exists(path + '.part')
 
 
+from PyQt6.QtWidgets import QApplication  # noqa: E402
+
+APP = QApplication.instance() or QApplication([])
+
+
+def test_compare_view():
+    before = QImage(10, 10, QImage.Format.Format_RGB32)
+    before.fill(QColor('red'))
+    after = QImage(40, 40, QImage.Format.Format_RGB32)
+    after.fill(QColor('blue'))
+    v = uq.CompareView()
+    v.resize(400, 400)   # at or above the minimum size
+    v.set_images(before, after)
+    shot = v.grab().toImage()
+    # left of the divider shows before, right shows after
+    assert shot.pixelColor(100, 200) == QColor('red')
+    assert shot.pixelColor(300, 200) == QColor('blue')
+
+
 if __name__ == '__main__':
     for name, fn in list(globals().items()):
         if name.startswith('test_'):