Just noticed something about the Operator Panel that takes up the lower part of the Tool Shelf. It seems it’s supposed to let me drag the boundary up to hide most of the upper tabbed panels, but not make them disappear completely. However, if I set the Display scaling to more than 1.04 (that’s the value that seems to be the critical one on my monitor), I can drag the boundary all the way up, but then it is the Operator panel that disappears, not the tabbed panels. Then a “+” icon appears at the bottom of the Tool Shelf, which presumably is clickable to bring the panel back -- only it does not work. I have to lower the display scaling to bring it back.
Contrast this with dragging the boundary down -- the Operator panel also disappears in this case, but the “+” icon that appears can indeed be clicked to bring it back.