Dragging scroll thumb moves window

[ryandesign]

Steps to reproduce

1. In System Preferences > General, set "Show scroll bars" to "Automatically based on mouse or trackpad" and attach a mouse, or set it to "Always"
2. Launch Image Shrinker 1.6.4
3. Convert a batch of several images (5 or more?) so that the results window has scrollable contents
4. Use the mouse to drag the scroll thumb

Result

The window moves instead of the scroll bar thumb, as if I had dragged the window's titlebar.