While a toolbar is expanded, hovering over the edge of a panel will dismiss the toolbar
MuseScore 3.5.2, on Windows 10, screen resolution 1920x1080, UI scaling 1.5x.
To reproduce: Snap a panel, such as the "Palette" panel, to the left-hand side of the workspace. Horizontally shrink the main window until the main toolbar's voice buttons ("1", "2", "3" and "4") disappear. Click the ">>" button to reveal the voice buttons. Move the mouse cursor towards the buttons.
Observed behaviour: When the mouse cursor lines up with the right-hand side of the panel, it changes into a "resize horizontally" symbol, and the voice buttons disappear.
Expected behaviour: UI widgets obscured by a popup should not be interactive.
In addition to this bug, the ">>" button seems to have a few ergonomic problems. The revealed buttons are left-aligned, which means that for a wide toolbar, they're always far away from the mouse; right-alignment would be more convenient. Moving the mouse cursor outside the boundaries of the popup will immediately dismiss it; this is inconsistent with other popups, like the main menu or the right-click context menu, which aren't dismissed unless the user clicks outside their boundaries. I'm unsure whether these problems are built into QToolBar, or whether they're unique to MuseScore.
Incidentally, the ">>" button's icon has a few graphical bugs: an unusually pixellated appearance, inconsistent vertical alignment, and the left side of the icon appears to have been clipped. Please see the attached screenshot.