Do usability tests!
- Select a photo, then "Open in external editor" in Shotwell; GIMP is configured.
- GIMP opens.
- ... and opens the photo in question.
- ... and opens "Welcome to GIMP" dialog with recent files, obscuring the photo I want to edit.
- select Colors/Curves. Tweak, hit OK.
- Next time I click on the window's contents, to bring it to the front, the Curves dialog appears, because apparently Curves is a tool.
- the Curves tool is not highlighted in the toolbar.