Crash when extending slur if system layout changes during operation

I think this is probably related to #36256: Layout shift on each layout - courtesy key signature calculation?, which uses essentially the same score. The crash here goes away if you insert enough line break to lock in the layout. The crash occurs while trying to work on the parent of a SlurSergment - which should be a System - but it turns out the segment has no parent. I'm betting that's because of that shift that keeps making systems appear and disappear.

I agree on the fact that this crash is directly related to the layout.
Even before he began to extend the slur, just do a double-click and press Shift: immediately a second page appears.

In particular, the custom size page in this example is involved: Custom size page, with: Width: 100.00 and Height: 125.00

If you return to the default size page (A4) - so, no more page break-, the behavior is correct.

With the fix for #36256: Layout shift on each layout - courtesy key signature calculation?, this is no longer reproducible with the posted score. But I am a little hesitant to simply close this, as I know the fix for #36256: Layout shift on each layout - courtesy key signature calculation? may not catch every possible case of layout shift, so there may be other situations where this same issue would be reproducible. It seems the code should be robust enough to handle a layout shift that occurs during the extension. But until we find another test case, it may not be worth worrying about.

Well, until we can reproduce it, no reason to keep it open.

Automatically closed -- issue fixed for 2 weeks with no activity.