Look at pictures bellow! If I (and suppose lot of people) am playing and composing and searching for instruments, I don’t see nonexistent borders where I can click with mouse. So, If you quickly click and release mouse button on a category column (with no borders), the category selection popup brings up un you accidentally select “None” category. Therefore selected instrument disappears from selected category.
I think that category selection popup should be opened after mouse button released. Programmatically it sounds - onMouseRelease, but not onMouseClick. In that case user should release mouse button and then click on category he needed to change for selected instrument.
Picture with mouse not clicked yet, but positioned on category column:
And picture with mouse button not released:
Maybe it happens only on small monitors and popup pops up in upper region of mouse position. But anyway it happens accidentally. Then I should search missing instrument in root list, if I remember a name of it.