fix #32866: score jumping issues

[MarcSabatella]

There are a bunch of related issues I am trying to fix together. The fixes are related enough that I think it best to make and test them together, but I will annotate the changes in the commentsto show which issue is actually fixed by which change.

The underlying problem here is that adjustCanvasPosition is called too often and/or with too broad a target. This is in part because Score objects emit posChanged signals any time you click a note:

MuseScore/libmscore/score.cpp

Line 3431 in 396c44f

emit posChanged(pos, unsigned(tick));

This causes all scoreviews to respond, when really, only the current scoreview usually needs to. Furthermore, there is not enough of the original context (eg, which note was clicked to trigger this) to be able to position the score well. I guess there isn't a more focused call to adjustCanvasPosition in order to maintain the separation between libmscore and mscore. Or may it is intended that in some cases, all scoreviews should respond. In any case, I elected to leave this alone, and instead deal with the problems on the scoreview side.

There are also other places where adjustCanvasPosition is called unnecessarily, or with to broad a target.

[MarcSabatella]

This was causing us to try to re-position the canvas to the beginning of the measure every time you clicked a note in a measure. But if the measure is too wide to fit on the canvas, that might actually force the clicked note off screen. This code was clearly (?) intended to only work during playback only, but it gets triggered when you click a note as well. See the video in this comment: http://musescore.org/en/node/31156#comment-131201

[MarcSabatella]

This addresses http://musescore.org/en/node/7724 - playback jumping if bottom staff of system is visible on screen. General fix is beyond me, but I can at least do a cheap fix for the case of a single system on the page. This is probably the most common case given this bug only strikes if the full system does not fit on screen - it means the system must be pretty big. The cheap fix is to just keep the same vertical position. Seems to work OK - better than nothing, anyhow.

[lasconic]

I'm not sure why this code is limited to a single system per page. My understanding is that it would work for two systems equally well?

[MarcSabatella]

I tried that :-). What happens is it literally keeps the canvas focuses at the same Y position the page, so even though the playback has moved on to the second system, the canvas stays focuses on the first. There is probably a way to calculate an appropriate offset into the the new system, but it wasn't obvious.

[lasconic]

Ok let's merge this. I have something just a little bit better maybe.

      // canvas is not as tall as system
      if (r.height() < showRect.height()) {
            if (sys->staves()->size() == 1 || !playBack) {
                  // track note if single staff
                  showRect.setY(p.y());
                  showRect.setHeight(el->height());
                  }
            else {
                  qreal min = qMax(showRect.y(), r.y());
                  qreal max = qMin(showRect.bottom(), r.bottom());
                  if (min + border < max) {
                        // let user control height
                        showRect.setY(r.y());
                        showRect.setHeight(r.height());
                        }
                  }
            }

[MarcSabatella]

I don't completely follow, but I tried this out, and I'm not sure I understand what is meant to be different about it. It still fails in the case as my code - page with >1 system. But your code looks cleaner - doesn't require separate cases - and in any event, as I mentioned before, the multiple-system-per-page really is not nearly as important as getting the single-system case right. And both versions do this.

BTW, in case it isn't totally clear what the problem case is: create a score with four staves per system and make sure you can fit at least two systems on the page. Zoom way in so only two or three staves fit on screen, position the canvas to view the last staff of the system, click a note on the last staff, and hit play. When the playback hits the end of the system, the canvas does jump to the next system as it should, but it jumps to the top of the next system rather than keeping the display focused on the bottom staff.

[lasconic]

mm that's how I tested it and it's working for me but only for the first page.

[MarcSabatella]

BTW, I can rebase if the PR otherwise looks good. I didn't want to mess up the line comments.