Keep one Mac window fixed while you rearrange the others

Pin a reference window with Pin Mode and let the remaining windows reflow around it.

A reference window is the one you keep looking at: a spec, a call, a chat, a preview. Pin Mode holds that window and reflows the others around it, so arranging the desk does not shove the reference away.

Pin, then arrange

Turn on Accessibility for Gablet, then quit and reopen the app if you have not already.
Pin the window that should stay.
Throw, snap, or restore a layout for the other windows. They reflow around the pinned one.

Unpin when that window should join the layout again.

A window is not a picture

Pin Mode works on a live window. If the reference is a screenshot rather than an app, pin the image with Shoverlay instead. The two pins solve different problems: one holds an app in the layout, the other holds a picture above other apps.

Get Gablet

Gablet is a menu-bar window manager for macOS 11.5 or later, and a Magnet and Moom alternative. You can download Gablet. The trial is 7 days, then a paid license. It does not track usage. Feature names in the app stay Quick Throw, Stash, and Pin Mode.