Mac swaps your monitors or moves the menu bar after sleep: how to stop it

With two or more screens, macOS sometimes forgets which one sits where, often after sleep or when you plug in again. The pointer leaves the wrong edge and the menu bar jumps to another screen. You can set it back in System Settings each time, or let Sashay put your arrangement back for you.

Why it happens

macOS tells screens apart by the details each one reports. Two monitors of the same model, or a dock that wakes them in a different order, can leave macOS unsure which is which, so it may put them back in the wrong places.

What macOS offers

Open System Settings, Displays, and click Arrange…. Drag the screens back to where they stand on your desk. To move the menu bar, drag it to another screen in the same view, or select that screen and set Use as to Main display. You have to do this again each time macOS swaps them.

Fix it with Sashay

Sashay learns the arrangement you set in System Settings and puts it back when macOS swaps your screens after a plug-in or a wake. The menu bar goes back to its screen too.

Download Sashay, drag it to Applications and open it.
Click Open Accessibility Settings and switch Sashay on.
With Sashay running, arrange your screens in System Settings, Displays, the way you want them. Sashay learns the arrangement when you change it there, so move a screen and put it back if it is already right.
In Sashay's Settings, under Layouts, leave Keep my screen arrangement on. It starts on.

From then on, when macOS swaps the screens, Sashay puts them back. It remembers one arrangement for each set of screens, so a desk with two monitors and a desk with one each keep their own.

If it doesn't work

It swapped and stayed swapped. Sashay only knows an arrangement you changed in System Settings while it was running. Set it once more with Sashay open.
The screens are mirrored. Sashay leaves mirrored screens alone.