It was already unchecked. The only difference that I noticed between 12 and 11.3 in General Preferences-Render was that Maxed Cache Renders was 200 for 12 and 100 for 11.3. I tried changing 200 to 100, but that did not solve the problem.

What I notice is that the document window is docking like the other palettes and the other palettes are docking like document window. So when docked, document window is squeezed off and the other palettes are eager to dock and take up half the page when they do.