Oh, one more thing. Try right-clicking on the tiny "resize grip" (usually three dots to the left of the window list) and in the menu that pops up, click "Preferences". Turn off window thumbnail previews in the preferences dialog that appears; that has also been known to cause some problems lately (the feature was only added in MATE 1.24, new to Ubuntu 20.04). If you ever find that you'd like compositing turned on, turn it back on and make sure you disabled window thumbnails in the Window List applet like I described above. Potentially, the thumbnails may be the problem (but just guesses -- I've had a few problems like this when the thumbnails appear too close to the panel buttons).
Other than that, adios, amigo!