Comment 4 for bug 1847479

No, you can't. The window EXPANDS to where it's unusable, but then doesn't resize back DOWN when you toggle details back off.

The only way out is to kill update-manager (which doesn't even have a Close icon, so you also have to know that you can do it from the main menu, which some distros are now disabling by default, so then you're looking at the terminal for it. :P)