I've seen a few posts about some or all apps suddenly are larger than the screen.
Had different or no fixes.

This is Mint 19.3 xfce on a laptop. Probably Intel graphics drivers.
It had been working fine, which makes me think of recent changes if it appeared suddenly.

1st noticed it on Firefox 80. Maximized, the Find bar Ctrl + F, was off the bottom of screen. The screen size & dpi under display settings are correct.

In xfce, I didn't find a "detect settings" under the display settings (as I see in Cinnamon).
Then I saw a setting to increase / decrease app windows "snapping to" display borders (I'm not at that PC now).

I increased its "snappiness", then dragged Fx's borders till it snapped against edges of monitor & I assume to the top edge of the panel (at screen bottom).
After restarting Fx a few times, it seems to retain that max size - & after reboot.

I don't know if the feature to increase snappiness is just for cases like this. I haven't tried every app that would ever be opened full screen, so don't know if that's a permanent fix for all apps - adjust them once to monitor edges & done.

I wonder if recent updates for X server could have "suddenly" caused this issue? I could uninstall them.

If not, any suggestions how to correct Linux's sudden amnesia of the display size?
In dconf editor, it still shows the same display 1400 x 900 & 96 dpi.

I will say the xfce total height of top toolbar / menu bar (wife uses it) & title bar are somewhat taller than on my Cinnamon machine. Her panel is also taller, but IMO neither is "excessive." Apps or the system are supposed to adjust to minor changes like that.

Usually, a right click on the title bar, & select maximise, will fill the screen, (even if it overlaps your actual screen at first).

@ fatmac, thanks. (I don't feel using derogatory nicknames. You're probably just "husky." Maybe if it was phatmac?)
You had me up until "(even if it overlaps your actual screen at first).

When does it stop overlapping your screen? After R-click titlebar > maximize, do you close the app? On reopening does it magically fit when maximized?
Or are we missing a step?

When you right click & select 'maximise', it should adjust to your actual screen size, & from then on use that size as its maximum size.

NB. I used 'fatmac' because I was getting overweight, so thought that it would be a good reminder.....I've lost about 22lbs since.