Why does this happen? It's driving me nuts.

[Lettuce Be]

1. Go to System Preferences Dock

Dock is in top row, Here are my settings:

Size nearly all the way to small
Magnification in the middle
Position bottom
Minimize using Genie effect
Prefer tabs when opening docs, Always

Then I have checked
Double-click a window's title bar to minimize
Minimize windows into application icon
Show indicators for open applications
Show recent applications in Dock

Try these settings and see if it fixes. Good luck! Stuff like this drives me batty.

Edit to add: If that doesn't fix things then check settings in Accessibility. That has keyboard shortcuts for zoom and maybe something is set that you actually use? You could just unclick to not have that be an option. There is also Zoom Style (mine has full screen, not even sure what it does)