"Set scale" thumbnail button shows a wrong image in the "set scale" window, and can leave the program in an inconsistent state

0. Run in debug mode.
1. Open p39.zip.
2. Select the "barbar" photo.
3. Click its "Set scale" thumbnail button. Unexpectedly, the Set scale window opens with the "a" photo shown:
4. Click Cancel. Unexpectedly, the "a" photo is now shown in the main window, but the "barbar" thumbnail seems to be selected: (Clicking on the "barbar" thumbnail doesn't correct the situation.)