Layout shift in measure at start of system

So far I can also say the page structure is relevant, I can't make a problem occur on the first page but can on subsequent pages. This appears to be because we start collecting the systems on the subsequent pages while still processing the first page, then realize too late the system wasn't going to fit on this page anyhow and roll back (the "rangeWasDone" code in collectPage). I'm currently thinking this code was never really working right (and there are comments in there from others expressing confusion as well), but that the problems only became exposed with my changes for https://github.com/musescore/MuseScore/pull/4866.

Further analysis: even though in theory we're done once we first encounter the measure that eventually gets messed up - the range is completed - we actually lay out that system again specifically because of the rollback of rangeDone. That rollback code was added just before release of 3.0, in https://github.com/musescore/MuseScore/pull/4373. I think I get why that added, but maybe this went too far, it shouldn't have forced more systems to be collected, just made sure we keep calculating system positions. And in fact, a fix I made recently for other system spacing issue might have made that rollback irrelevant. At least, I can't reproduce the original problem from #279367: First system on a page may not fit to the page even if I remove the rollback, and I suspect it's because my changes in https://github.com/musescore/MuseScore/pull/5051

More testing is needed to verify there are no other dependencies here, and in any case, even if it wasn't strictly necessary to lay out the system containing the messed up measure, it also shouldn't have hurt anything. Somehow we're getting the width of the measure wrong on this relayout, presumably having something to do with all that adding and removing of the header that goes on while adding measures to the system and somehow the key signature segment getting the wrong width in the process. So I'd like to understand that as well, and then hopefully submit a PR with both fixes.

I've figured out why the key signature width changes from the first layout to the second. It's because of the measure rest on the staff with the wider key signature. The first we layout, it hasn't been centered yet, and we calculate a minimum distance from the key signature to that rest, which ensures that all notes and rests on all staves start after the widest key signature. But by the next layout of the measure, the rest has been centered, so we no longer think we need to allocate space to clear the key signature for that staff.

So, the full set of conditions for the bug to appear:

1) multiple staves with different key signatures
2) >1 page (issue will only show after first page)
3) full measure rest on the first measure of a system, for staff with one of the widest key signatures

I do want to revert the rangeDone rollback so we don't layout the system in question unnecessarily to begin with. But since I can't guarantee there will never be multiple layouts of the same system, I plan to deal with the full measure rest (will check repeat measure too).

I've filed a related issue #289982: Notes on staff with no/narrow key signature can overlap key signature on other staves to log why it is good that I am dealing with the full measure rest issue even though it isn't needed here if I also revert the rangeDone rollback. I managed to force something else to go wrong if I don't include this code. I'll make that a separate commit on the PR for better tracking.

https://github.com/musescore/MuseScore/pull/5081