If you use Ubuntu MATE you may have found that it’s really difficult to resize windows; you can move your mouse pointer over the edge of a window to drag the window bigger or smaller, but the “grab area” is only one pixel wide. This is alaarmingly irritating.
It’s a long-standing bug (people were complaining about this in 2010, seven years ago!), which has been fixed in Ubuntu proper for a long time, but has resurfaced in Ubuntu MATE. Anyway, to fix resizing of windows, you need to enable Compiz in Ubuntu MATE; that replaces Ubuntu MATE’s standard “window manager”, which is named “Marco”, with the Compiz window manager, and Compiz doesn’t have this daft problem. Yes, it is annoying that this doesn’t just work, and apparently they’re working on fixing it so it doesn’t become our problem as users to fix the deficiencies, but in the interim you can at least fix it for yourself even though you shouldn’t have to.
So, open MATE Tweak, which is in the System menu (Preferences > Look and Feel > MATE Tweak), and under Windows, choose Compiz (Advanced GPU accelerated desktop effects) under Window Manager.
Technically, Compiz requires a 3d accelerated graphics card. However, unless your machine is very, very, very old indeed, it will have enough 3d acceleration to do this; this is not like playing games or similar. My ancient Dell laptop copes with it fine, so it should not be a worry.
Ubuntu MATE will then switch to Compiz (you don’t need to reboot or anything) and will show you a window saying “Keep this window manager?” If you see that window, you can click “Yes, OK” in it. (If for some reason this hasn’t worked, then you won’t see that window and so it will automatically switch back, so your computer isn’t broken.) Now, resizing Ubuntu MATE windows should be a lot easier, because the resize grab area will not be one single pixel.