How can I access windows that become inaccessible when using two displays?
As a workaround, rearranging the displays allows temporary access to the hidden windows. The windows can then be moved to (hopefully) make them accessible with the original display arrangement.


Open System Preferences
Open the "Displays" preferences
Click "Arrangement" tab
Move 2ndary display around primary display until the missing windows appear.
Move the (formerly missing) window to the primary display.
Move the 2ndary display back to the original position.


After restoring the display arrangement, the window may become inaccessible again; steps 4-6 may need to be repeated.