Main window becomes tiny after being maximized
In OS X, there are three buttons on the top left corner of a window. The red one closes the window, and the yellow one minimizes it. The green one's function has changed slightly with the latest OS X version—before, it sized the window to fit the screen, an entirely separate function from the full screen button on the top right; in OS X Yosemite and later, the green button is the full screen button, but its old functionality can be accessed by holding down [Option] while clicking it. The bug arises when the button is used to fit the window to the screen.
Quite simply, if you do that, then the next time you open MuseScore the window will be tiny. Like this (actual size after exercising the bug):
This may only be an issue with OS X—I don't know if other operating systems provide an option to size a window to fit the screen (not to be confused with full screen).