I have updated to version 11.3 of Mathematica, and am now getting a bizarre problem with the palettes and some of my notebook files. The contents are shrunk relative to the window size. See below.
If I change the zoom level (bottom left), the problem just scales up. The action of the mouse cursor seems displaced from the actual location of the mouse cursor, as though the contents of the window were correct. For example, if I place the cursor where a particular palette element SHOULD be, the tooltip for that element pops up.