Fairly fresh install of MATE 18.04 in MBP '09 . . . overall working well, but lately noticing that the bottom panel will “flicker” when scrolling down through some web page, or, also as a web page is loading . . . once the page has loaded the flickering stops . . . . Seemed like if more than one app is loaded and showing in the panel the problem is “less” . . . . It’s not a show stopper, but, somewhat irritating . . . visually distracting.
I saw another thread entitled “Panel flickering” in 16.04, and I read through it, some of the posters were mentioning “window compositor” in MATE Tweaks . . . but I couldn’t exactly figure out what they wanted the OP to do with it . . . .
Window Manager was originally set to "Marco (adaptive compositor) . . . which is possibly the default, I don’t think I changed it or set it to that . . . . I have another install of U-MATE 18.04 on a desktop computer, but the problem does not seem to affect that computer . . . I did change the WM on that install to Marco (Compton) . . . after that suggestion was made as a way to solve the problem on the laptop . . . .
This issue was posted on the U-MATE forum and it brought the suggestion to try to change the WM to Marco (Compton) which helped the poster and one other individual . . . but doing that change did not solve the issue . . . where each key stroke makes the panel “shake” . . . or scrolling will "split" the panel horizontally . . . . I've done a few updates/upgrades to the system since the install--problem persists.
Noticed that the problem was happening while using the Terminal as well. As an update/upgrade was running the panel was flickering . . .