Clicking buttons does not work if mouse moves during the click

When you say you contacted "support", please realize there is no official "support" for MuseScore - I guess maybe you tried to contact the folks at the commercial score-sharing website musescore.com? They generally don't know anything about the software. The way to get support is to post to the forum here.

Sounds like perhaps a dialog box is appearing but is off-screen for some reason. Is this your first time starting MuseScore 4? If so you should be seeing a window asking you some questions, like what color theme you want to use. Has that already appeared? Try pressing Esc to close the dialog even though you can't see it.

Hmm, I thought at first you said it was starting up where it froze, but now it sounds like you are saying the freeze happens after startup and selecting instruments? Can you give precise steps to reproduce the problem - starting after the program starts up, and leading up to the point where it freezes? A video could definitely be helpful as well. You can upload your video to YouTube (make it unlisted) then post the link here.

It looks like you are missing a step between 4 & 5. After doing a search, you are presented with potentially multiple results. You need to then select one of those search results before the add to score button is active. And yes, double-clicking an instrument form within the search results is the same as first selecting one of the results and then clicking the add to score button. And yes, until you select something from within the search results, the add to score button is disabled, because you have nothing in your score yet.

But clicking the disabled add to score button shouldn't lock up the window, now does it for me. It's still just waiting for me to actually select something to add to the score.

Unfortunately your video doesn't appear to be related at all, it doesn't show the dialog. You probably need to tell your screen recorder to do "entire screen" instead of just the main application window.

Thanks for the updated video and the explanation! From what I can tell, the issue is that your mouse is moving while clicking - a plain click does work. But I can confirm that if I am moving my mouse slightly while I click, things don't work. That's something we can look at improving at our end. Meanwhile, you might check to see if you can reduce the sensitivity of the mouse to small motions - there might be a setting either in the mouse driver itself or in your systens accessibility settings.

You might also consider making more use of keyboard navigation & shortcuts. For instance, after typing "piano" into the search box, use the Down key to move focus into the list, hit Enter. Then Tab to reach the bottom row of buttons, cursor keys to move between them, etc. Similarly for other controls - they are all generally reachable by keyboard, which is a specific accessibility feature intended for people who have difficulty with the mouse (especially blind users for whom the mosue is more or less completely not an option). That said, it's certainly the mouse interaction can be improved as well.

I've updated the here to be more descriptive of what I believe the issue to be. If you can confirm this seems accurate (maybe have someone else try this on your computer to be sure your mouse itself isn't simply defective, but I assume it's really more releated to the disability you explained), then we can open an issue on GitHub where the developers can investigate possibile solutions.