[MU4 Issue] Tempo lines are invisible if there's nothing on those beats in the top staff

[erinic04]

Describe the bug
Tempo lines are invisible if there isn't anything on the beats where the tempo lines are in the top staff (eg. last two beats of a whole note)

To Reproduce
Steps to reproduce the behavior:

1. Create a score
2. Put tempo line on last two beats of measure
3. Put whole note on top staff
4. See no tempo line

Expected behavior
The tempo line should still be there

Screenshots

Platform information

• OS: [e.g. macOS, Windows, Linux]

Additional context
Add any other context about the problem here.

[oktophonie]

This is a general problem, it seems, when the rhythmic attachment point disappears. For example, taking this:

and changing the first half note to a whole note causes the hairpin to vanish:

While we desperately want to be able to attach things to specific rhythmic positions within notes, that's a future (4.x) feature, so the question is what the behaviour should be right now. @Tantacrul

[Tantacrul]

The only options I can think of are:

1. Leave as is
2. Line becomes attached to the note that 'overlapped' the note it was originally attached to. However, this seems so weird, I worry there'd be follow-on consequences.

This is a MS3 problem too though right? In which case, I don't think we should fix for 4.0. Better to fix properly.

[oktophonie]

The problem with lines exists in 3.6; tempo lines are new in 4.0 so on the one hand it's just another instance of the same problem, but on the other hand it's 'new'.

In 3.6, however, a plain tempo (not a line) does keep its position in this situation. This:

becomes this:

It would be great if lines could do the same. I suspect that's infeasible for now, but we'll have a look in case.

[Tantacrul]

Yeah, why not see? Thanks @oktophonie :)

[mike-spa]

Closing this issue as it's been incorporated into the more general #14296. Will be fixed by #14397.