Drop the comparison slider, drag the zoom point, allow 8 steps

geometry.py

@@ -28,10 +28,17 @@ def zoom_window(img, upscale, center=(0.5, 0.5)):
 
 
 def nested_rects(size, levels, upscale=4, center=(0.5, 0.5)):
-    """Where each zoom step lands, in the coordinates of the first canvas."""
+    """Where each zoom step lands, in the coordinates of the first canvas.
+
+    Stops early once a window would be under a pixel wide. The recursion itself never
+    degenerates, because every step resizes its crop back up to the full canvas, but drawing
+    all those steps onto the one original canvas does run out of pixels.
+    """
     rects, rect = [], (0, 0, size[0], size[1])
     for _ in range(levels):
         side = rect[2] - rect[0]
+        if side // upscale < 1:
+            break
         l, t, r, b = window_rect((side, side), upscale, center)
         rect = (rect[0] + l, rect[1] + t, rect[0] + r, rect[1] + b)
         rects.append(rect)