Press and hold left alt key, then you can click and drag windows around from anywhere (as opposed to having to click and hold top bar to move windows)
I see a lot of folks on a fresh install saying this doesnt work anymore. Keep in mind this is not simply an Rpi feature, it's linux, Xorg to be more specific, and has been around longer than Rpi's have. Needless to say... it still works.
A likely cause, especially to those newer to linux, is your locale and keyboard layout settings. If these setting don't match your situation, you will run into a handful of annoying little problems like this where you press the alt key for example, but the system registers it as some other key. (if you have already checked this and know you're configured properly, the rest if this post won't help you)
A fresh Raspian install locale defaults to the UK with a generic UK keyboard layout. I'm in the US so I'll run through a quick US config, but this should still help those from elsewhere get pointed in the right direction.
Boot up, log in terminal style and type: sudo raspi-config
Navigate to: Internationalisation > Change Locale
Spacebar to uncheck the en_GB.UTF8 option and then down a bit further to select en_US.UTF-8 UTF-8 instead. < ok> and back out to main menu.
This time to: Internationalisation > Change Keyboard Layout
In here you want Generic 104-key PC and then English (US). It will ask you a few additional preferences, you can play it safe go with the defaults if you're a little unsure on these.
Back out, reboot into GUI, left alt + left click and hold anywhere on a window to drag it around.
Somewhat unrelated tip... If you get sick of typing sudo every time you're in the terminal doing root stuff type: sudo -i and hit enter. This will put you in "perma-root mode" for the remainder of that session, or until you exit back out of it.
EDIT: Another instant workaround that might even work with the UK/GB settings is to try right alt. I can't confirm this as I am using one of them little compact wireless keyboards which literally does not have a right alt key, but if you're on a full size it's worth a shot.