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>

3 files changed, 120 insertions, 4 deletions

diff --git a/docs/superpowers/plans/2026-10-30-upscale-qt.md b/docs/superpowers/plans/2026-10-30-upscale-qt.md
index 9af7600..8254308 100644
--- a/docs/superpowers/plans/2026-10-30-upscale-qt.md
+++ b/docs/superpowers/plans/2026-10-30-upscale-qt.md
@@ -241,12 +241,12 @@ def test_compare_view():
     after = QImage(40, 40, QImage.Format.Format_RGB32)
     after.fill(QColor('blue'))
     v = uq.CompareView()
-    v.resize(200, 200)
+    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(50, 100) == QColor('red')
-    assert shot.pixelColor(150, 100) == QColor('blue')
+    assert shot.pixelColor(100, 200) == QColor('red')
+    assert shot.pixelColor(300, 200) == QColor('blue')
 ```
 
 - [ ] **Step 2: Run, expect failure**
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_'):
diff --git a/upscale-qt b/upscale-qt
index 5baa355..41e3f5a 100755
--- a/upscale-qt
+++ b/upscale-qt
@@ -19,7 +19,9 @@ import os
 import sys
 from pathlib import Path
 
-from PyQt6.QtGui import QImage, QImageReader
+from PyQt6.QtCore import QPointF, QRectF, Qt
+from PyQt6.QtGui import QColor, QImage, QImageReader, QPainter, QPen
+from PyQt6.QtWidgets import QWidget
 
 MODEL_EXTS = ('.pth', '.safetensors', '.gguf')
 JPEG_QUALITY = 95
@@ -70,3 +72,98 @@ def save_over(img, path):
         return False
     os.replace(part, path)
     return True
+
+
+class CompareView(QWidget):
+    """Before left of a draggable divider, after right, one shared transform.
+
+    Before is drawn into after's rectangle, so it is scaled on paint, never
+    copied at 4x size."""
+
+    GRAB = 8  # px around the divider that start a divider drag
+
+    def __init__(self):
+        super().__init__()
+        self.before = self.after = None
+        self.split = 0.5
+        self.scale = 1.0
+        self.offset = QPointF()
+        self.drag = None
+        self.setMouseTracking(True)
+        self.setMinimumSize(400, 300)
+
+    def set_images(self, before, after=None):
+        self.before, self.after = before, after
+        self.fit()
+
+    def ref(self):
+        return self.after if self.after is not None else self.before
+
+    def fit(self):
+        img = self.ref()
+        if img is not None:
+            self.scale = min(self.width() / img.width(),
+                             self.height() / img.height())
+            self.offset = QPointF((self.width() - img.width() * self.scale) / 2,
+                                  (self.height() - img.height() * self.scale) / 2)
+        self.update()
+
+    def divider_x(self):
+        return self.split * self.width()
+
+    def paintEvent(self, e):
+        p = QPainter(self)
+        p.fillRect(self.rect(), QColor(40, 40, 40))
+        img = self.ref()
+        if img is None:
+            return
+        if self.scale >= 0.5:
+            # ponytail: smooth only near 1:1, smoothing a 160 MP image
+            # scaled down to screen size stalls the paint
+            p.setRenderHint(QPainter.RenderHint.SmoothPixmapTransform)
+        target = QRectF(self.offset.x(), self.offset.y(),
+                        img.width() * self.scale, img.height() * self.scale)
+        if self.after is None:
+            p.drawImage(target, self.before)
+            return
+        x = self.divider_x()
+        p.setClipRect(QRectF(0, 0, x, self.height()))
+        p.drawImage(target, self.before)
+        p.setClipRect(QRectF(x, 0, self.width() - x, self.height()))
+        p.drawImage(target, self.after)
+        p.setClipping(False)
+        p.setPen(QPen(QColor('white'), 2))
+        p.drawLine(QPointF(x, 0), QPointF(x, self.height()))
+
+    def near_divider(self, pos):
+        return self.after is not None and abs(pos.x() - self.divider_x()) < self.GRAB
+
+    def mousePressEvent(self, e):
+        self.drag = 'split' if self.near_divider(e.position()) else e.position()
+
+    def mouseMoveEvent(self, e):
+        pos = e.position()
+        if self.drag == 'split':
+            self.split = min(max(pos.x() / self.width(), 0.0), 1.0)
+        elif self.drag is not None:
+            self.offset += pos - self.drag
+            self.drag = pos
+        self.setCursor(Qt.CursorShape.SplitHCursor if self.drag == 'split'
+                       or self.near_divider(pos) else Qt.CursorShape.ArrowCursor)
+        self.update()
+
+    def mouseReleaseEvent(self, e):
+        self.drag = None
+
+    def mouseDoubleClickEvent(self, e):
+        self.fit()
+
+    def wheelEvent(self, e):
+        f = 1.25 if e.angleDelta().y() > 0 else 0.8
+        pos = e.position()
+        self.offset = pos - (pos - self.offset) * f   # zoom around the cursor
+        self.scale *= f
+        self.update()
+
+    def resizeEvent(self, e):
+        self.fit()  # ponytail: resize resets zoom, keep the view center if it annoys