Grey screen when loading next image

Is there an existing issue for this?

• I checked and did not find my issue in the already reported ones

Describe the bug

Same bug as https://github.com/darktable-org/darktable/issues/20030, which got closed. It also happens on my MacBook Pro (M5, Tahoe 26.5.2). It does not seem to be packaging or Wayland-specific issue, like I first suspected.

Back in 5.2.1, when toggling show loading screen between images off, the previous image would stay visible as long as the next image was loading (rather than showing a dark grey screen). Seems that behaviour is now broken.
I'm running Darktable 5.6.0 on macOS 26.5.2 with OpenCL disabled (that's the default, I believe, but the behavior happens with it enabled too).

Steps to reproduce

1. Go to the darkroom menu inside preferences
2. Toggle show loading screen between images off
3. Enter darkroom and go back and forth

Expected behavior

I should see the previous image while the next one loads.

Logfile | Screenshot | Screencast

Same recording as before, as the behaviour is entirely identical.

https://github.com/user-attachments/assets/6a72ea36-c23b-4c9f-bbaf-e46c4b26ac11

Commit

No response

Where did you obtain darktable from?

darktable.org / GitHub release

darktable version

5.6.0

What OS are you using?

Mac

What is the version of your OS?

macOS 25.6.2

Describe your system

No response

Are you using OpenCL GPU in darktable?

None

If yes, what is the GPU card and driver?

No response

Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip

No response

Reproduce the problem from Preferences > darkroom with “show loading screen between images” disabled, then navigate back and forth while comparing the 5.6.0 behavior with the expected previous-image display. Trace the darkroom image-loading and screen-transition handling, and verify that the previous image remains visible until the next image is ready on macOS.