Tempo line click and drag to beginning of score crash

[zacjansheski]

Issue type

Crash or freeze

Bug description

If you click/drag the tail of a tempo line and send it to the beginning of the score, MuseScore crashes.

In Master, not 4.3.1

Steps to reproduce

1. Make a score with a tempo line (rit.../acel.../etc)
2. Click and drag the end of the line to the beginning of the score
3. Crash

Screenshots/Screen recordings

2024-05-21.16-48-05.mp4

MuseScore Version

OS: Windows 11 Version 2009 or later, Arch.: x86_64, MuseScore Studio version (64-bit): 4.4.0-241420305, revision: github-musescore-musescore-449a64e

Regression

Yes, this used to work in a previous version of MuseScore 4.x

Operating system

All

Additional context

TempoLineCrash.zip

[mathesoncalum]

Alright, I’ve investigated this but haven’t found a satisfying fix just yet. Posting my findings for now, will return to this later with fresh eyes.



The moment the right grip is dragged beyond the problematic point, the LineSegment is put into some sort of invalid state and disappears. This isn’t actually the point where crash happens - if you’re quick enough you can release the drag and you’re left with an invisible line segment (which can be brought back if you insert a measure before the first).

We then get a layout call which resets the explicit parent. Whatever the “invalid state” is exactly (I'm still figuring this out), it prevents the LineSegment from being re-assigned an explicit parent during the layout. This is eventually what causes the crash at the next drag event (sys is null in findSegmentForGrip due to no explicit parent).

[bkunda]

I can no longer reproduce this in the latest nightly.
@zacjansheski @mathesoncalum could you perhaps please cross-check and verify?