Working between two windows or tabs

My MBAir (Sequoia 15.3.2) just got back from the shop and it's working OK, *BUT* I can no longer have two windows or tabs open on the same desktop, whether its the main desktop or a secondary one.

I often want to work between two or more documents at the same time.... cut/paste, compare changes, etc.

HELP!

What can I do to not have documents, folders, images, contact entries, etc. jump all over the screens?!?

I already have (real) vertigo. This is driving me to migraine headaches!