fix #289562: add missing blockSignals() calls on updating Mixer controls #5060
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Fixes https://musescore.org/en/node/289562, or, at least, the symptoms of this issue.
Why this fix works
Not blocking signals from port and channel sliders on updating their values made
instrumentsChangedflag in MasterScore be set totrueevery time these values were updated.MuseScore::endCmd()triggers such an update every time if both Mixer exists and that flag is set to true. Fortunately a bit laterendCmdsets this flag to false again but this doesn't manage to happen if the score has just been opened. The workaround by pressing Escape button before adding an instrument mentioned here works exactly because it makesendCmd()be called and, thus, resets the mentioned flag tofalse.Why the issue appeared only in 3.1-RC
The issue became much more visible after merging #5011 which caused Mixer object be created regardless of being actually displayed. In fact, the issue exists at least from 3.0.4 version (probably 3.0.3 but I tested it only on 3.0.4). In order to reproduce the issue on the versions before 3.1-RC one should open a Mixer window before executing steps 4-5 from this comment.
Although this fix works the more fundamental problem is probably having a somewhat invalid instruments and channels state at the moment when
MuseScore::endCmd()gets executed on updating a score here. Without that the signals blocking issue fixed here would cause only a small negative performance effect rather than a crash. This more fundamental issue should probably be addressed later but this patch should also be good to apply regardless of that.