Virtualbox 6.1.36 r152435, Host Win7, Guest Win7, Guest Addons installed, Guest graphics controller set to recommended VBoxSVGA. Host has two screens, each set to 1920x1200. Resizing the VM window does resize the guest screen, maximizing the VM window does not. It worked fine in Virtualbox 5. Tried to switch to VBoxVGA, did not help. What's wrong?
Update: Guest Resolution selection ends at 1400x1050. That explains why it can't switch to 1920x1200, but why the limit?
Reinstalled guest addons, switched to single VM monitor, resizing works after serveral reboots. Switching to two VM monitors breaks it again. Increasing VM graphics memory from 64 MB (indicated green in the settings screen!) to 128 MB finally resolved the issue.