Cannot right-click on Taskbar to display pop-up menu due to taskbar is full.

Cause:

Normally you can click on an empty area of the Taskbar to display its pop-up menu, but the Taskbar has no empty areas due to a large number of open applications.

Solution:

Move the mouse pointer to the edge of the Taskbar (not on a button) and right-click. This is useful when the Taskbar is full of buttons.