SUMMARY If the Mean Removal or Sharpen filters are used in a Filter Layer or Filter Mask, they produce visual artifacts in the shape of square tile outlines. STEPS TO REPRODUCE 1. Add a new Filter Layer or Mask 2. Pick Mean Removal or Sharpen from the Enhance category OBSERVED RESULT Visual tiling artifacts appear on the canvas. The artifacts persist when the filter layer is flattened or merged. EXPECTED RESULT No such artifacts. SOFTWARE/OS VERSIONS Tested with 4.3.0-prealpha (git 511ecc7) and 4.2.6. These artifacts only appear in master (both my build and the latest appimage), not in 4.2.6.
On Windows 10, I can confirm that this happens with the latest 4.3.0-prealpha (git 511ecc7) and with 4.2.7-beta1 but not with 4.2.6 The artifacts are a 500 x 500 px grid, for a range of different image sizes. Setting to Confirmed
Additional Note: This may (or may not) be related but with the Artistic -> Raindrops filter, the result shows distinct bulk tiling of 500 x 500 px. It also takes a very long time to do it. If you then try to put a filter mask on any layer, the Raindrops filter is selected and starts to be applied. If you quickly select a different filter, you have to wait for the Raindrops filter to be 'cancelled' (30 seconds in my case) before the newly selected filter is used. I see this on Linux appimages from 4.2.5 to the current latest 4.3.0 prealpha.
It also happens with Emboss filters(if those are used in a Filter Layer or Filter Mask).
*** This bug has been marked as a duplicate of bug 411159 ***