Black Spots in Lighttable Previews

Is there an existing issue for this?

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

Describe the bug

After editing some of my photos, the image shows up in the light-table preview with black spots in various places. It appears to be linked to both the number of images displayed per row in lighttable, and the style applied. On my system, it only occurs when the preview image is smallish (maybe under 400px in displayed width?)

I have tested it on both a 2560x1440 display (where the spots show up at 5 images per row, but not 4) and on a 1920x1080 display (where they show up at 3 images per row, but not 2).

This only occurs with certain modules active - with the default set it doesn't occur, but when applying surface blur I can get it to occur fairly regularly.

Steps to reproduce

1. Start 5.2.1 on Windows 11, with three screens enabled (2x 1920x1080, 1x 2560x1440)
2. Go to lighttable, select a NEF from a Nikon D7200, and open in darkroom.
3. Apply surface blur to a chosen NEF image (a stronger application should make it more obvious). I chose radius 23px, RGB sliders all set above 0.03.
4. Return to lighttable, and ensure view is set to filemanager.
5. Adjust number of images per row using the +/- buttons
6. Observe that for certain large numbers of previews per row, there are spots on the preview that are not present either when there are lower numbers of previews per row, or in the darkroom view.

Expected behavior

Preview image should match darkroom view regardless of zoom scale in the lighttable.

Logfile | Screenshot | Screencast

No response

Commit

No response

Where did you obtain darktable from?

darktable.org / GitHub release

darktable version

5.2.1

What OS are you using?

Windows

What is the version of your OS?

Windows 11, 25H2 version 10.0.26200

Describe your system

Lenovo x64 with i5-13450HX
16 GB Ram
8 GB GPU

Are you using OpenCL GPU in darktable?

Yes

If yes, what is the GPU card and driver?

NVIDIA GeForce RTX 4060, 8GB, NVIDIA 32.0.15.7262

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

No response

Start by reproducing the issue in the lighttable filemanager view with a surface-blur-edited NEF, varying the number of previews per row and comparing it with darkroom. Trace the lighttable preview rendering and OpenCL path; done means previews remain free of black spots and match the darkroom image at the affected scales.