Bug 491276

Created attachment 172282 [details]
instant preview canvas glitch after move

SUMMARY
On a large canvas, tools that use Instant Preview cause artifacting and glitchy canvas rendering when returning to a non-Instant Preview state, after the zoom level has changed significantly during the operation, and the tool/operation is not "properly" committed/ended. It's most noticeable near bottom of canvas, but applies elsewhere too.

STEPS TO REPRODUCE
1. Ensure "Force Instant Preview for Move Tool" is enabled under Configure Krita > Performance > Instant Preview.
2. Create a larger document: A3 300ppi (3508 x 4960).
3. Zoom out so the canvas is small (< 10%).
4. Fill the background color with a light grey.
5. Make a new Paint Layer and draw some black squiggles along the bottom edge of the canvas.
6. Make another new Paint Layer and draw a horizontal line in the center of the canvas, crossing the whole width. The line is better off not perfectly straight -- use a regular brush.
7. Select the Move tool on the horizontal line's layer.
8. Click and drag to move the layer towards the bottom of the canvas, then back up to the center near where it was originally. **This is necessary.** 
9. A green box should appear around the layer to signify the Move tool is still active.
10. Zoom in towards the bottom of the canvas (>40%). Notice that Paint Layer 1 (squiggles) appears very fuzzy from the Instant Preview, and a white line is showing along the entire bottom edge of the canvas (despite the background being grey). 
**Sometimes the white line shows up on the side of the canvas instead. I don't make the rules...**
11. Without pressing Enter or Escape, either:
  11a. Select another tool (e.g. brush tool); or
  11b. Select another Paint Layer.

OBSERVED RESULT
The area around the bottom of the canvas (where the horizontal line was moved to, then moved away from) will be stuck in a low-detail Instant Preview state, while only the area near where the horizontal line layer was moved back to will be reverted back to normal. The bottom of the canvas will remain this way until something happens to update it. Drawing a line will only fix the area immediately around the brushstroke. The solution seems to be to toggle the background layer(s).

EXPECTED RESULT
The entire canvas should be reset back to a fully-rendered, not Instant-Preview state.

SOFTWARE/OS VERSIONS
Windows: 10 & 11
Linux/KDE Plasma: Linux Mint 21.3 Cinnamon
Krita: 5.2.2 & 5.2.3 (Windows installer, Linux AppImage)

ADDITIONAL INFORMATION
I've attached a gallery of screenshots showing what it looks like. 

I've tested and observed this behavior on multiple computers (desktop/laptop), between multiple OS' (Windows/Linux), dedicated/integrated graphics, drawing tablet/mouse/trackpad, and every Krita 5.2.x build I've tried. This happens on new installs of Krita, as well as on my main system. 

It seems mostly related to the Move Tool and the Transform Tool, so long as Force Instant Preview is toggled on for that tool; the regular Instant Preview Mode (View > Instant Preview Mode) being toggled on/off during normal use seems irrelevant.

This might be related to bug 480973 but I think that one might be specific to the Transform Tool and its "Preview" mode Tool Options.