Redrawing of Navigator for large scores causes more lag than in MS3

[dxing97]

(Updated after #19639, which has improved the situation a lot when only the blue rectangle inside the Navigator has to be updated, but still very laggy when the whole Navigator needs to be redrawn)

Describe the bug
If the navigator is open on a score with many measures, parts, and/or notes, the frame rate drops significantly and scrolling across the score becomes difficult. If the score is large/complex enough, it can become unusably slow.

To Reproduce
Steps to reproduce the behavior:

1. Create a new score using the Orchestral -> Symphony Orchestra template
2. Click on Add->Measures->Append Measures
3. Append 500 measures.
   Scrolling around zoomed in is very smooth. If zoomed out such that the entire score fits in the workspace, scrolling becomes slow.
4. Click on View -> Navigator
   When the Navigator has to be redrawn (for example, when its viewport changes), MS4 lags more than MS3.

Expected behavior
Smooth scrolling when not zoomed out on a large score with the navigator open.

Additional context
Also present in MuseScore 3.6.2. Can occur with significantly fewer measures if there are many notes/elements on the staff.

[bluebear94]

Commenting out the emit viewportChanged(); line in AbstractNotationPaintView::onMatrixChanged removes the lag. The bottleneck is probably there.

[bluebear94]

Just noticed the part of the profiler that shows profiling info for other threads. Actually looks like the navigator is repainting itself excessively.

[bluebear94]

Actually, MS3 seems to redraw the navigator on every update, so why does MS4 take a lot longer to do that?

Edit: from observing MS3 behavior, I’ve noticed that dragging the rectangle in a way that causes the navigator to scroll is less smooth than otherwise, so not repainting on every update might have to do with it.

[bluebear94]

This issue is only partially solved. When the navigator viewport changes, then redrawing the contents is slower in MS4 than in MS3.

[cbjeukendrup]

Reopening because of the comment above