Overlay scrollbar intercepts clicks on selection handles at image edges when zoomed in

[AoMas17]

Description
When a rectangle selection (or any selection made with the Rectangle Select or Ellipse Select tool) has one of its edges touching or near the boundary of the image, and the canvas is zoomed in enough that scrollbars appear, GTK's overlay scrollbar appears on top of the selection resize handle. Clicking in that area activates the scrollbar instead of grabbing the handle, making it impossible to resize the selection from that edge.

To Reproduce

1. Zoom in until the canvas scrollbars appear.
2. Draw a rectangle selection that reaches the right or bottom edge of the image.
3. Move the cursor over a resize handle on that edge.
4. Try to click and drag the handle to resize the selection.

Expected behavior:
The resize handle receives the click and the selection can be resized normally, regardless of whether a scrollbar is visible in that area.

Actual behavior:
The overlay scrollbar is drawn on top of the resize handle and intercepts the pointer event. The handle cannot be grabbed. The only way to work around it is to zoom out until the scrollbars disappear, adjust the selection, then zoom back in.

Additional Info

• The problem only occurs when the canvas is zoomed in enough to show scrollbars, because that is when GTK renders the overlay scrollbars over the canvas content.
• A short video demonstrating the issue is attached.

Version

• Pinta version: 3.1.2
• OS: macOS Tahoe Version 26.5.1 (25F80)
• GTK version: 4.22.4

Screen.Recording.mp4