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Put your windows back when a monitor reconnects

The external display is unplugged and everything piles onto the laptop; it is plugged back in and a rule puts all four windows back.

Set it up once

1. With the monitor connected, arrange your windows on it the way you want them. Keep Hammerdeck's own windows on the laptop screen, so they aren't captured too.
2. Click Hammerdeck's menu bar icon, choose Rules…, and pick the recipe Arrange windows on dock (“Snap your windows into place when a display connects”).
3. The rule reads “When display connects”. Click the display name and make sure it is your external monitor, not the laptop's built-in screen: the next step captures only windows on the display named here.
4. Click Capture current layout. Every window on that monitor is recorded: its app, and its exact position and size.
5. Click Add rule.

From then on, each time that monitor connects, by cable or through a dock, the captured windows go back onto it in the same places. To try it without unplugging, drag a couple of those windows onto the laptop screen, then click the ▶ next to the rule in the list. With several monitors, make one rule per display.

What to expect

• Only windows of the apps you captured are moved; everything else stays where it is.
• Windows are matched by app name. Each captured window is one slot, filled by that app's windows from front to back, so two windows of the same app can trade places. To pin one, click Edit as JSON and put part of its title in that window's titlePattern.
• Apps that are not running are skipped, and the run says which ones in Hammerdeck's local log, along with any window macOS refused to move.
• A rule can do more than arrange windows. The same trigger can open an app, set the wallpaper, or run any macOS Shortcut, in a chain.
• Moving windows needs the Accessibility permission; Hammerdeck explains why on first launch before macOS asks.

And by hand, in between

For the moves you make all day, ⌃⌥⌘] throws a window to the next display and ⌃⌥⌘Z undoes the last move. More in moving a window to another display.

Other ways to do it

• Moom ($15) can apply a saved layout automatically when displays are added or removed, and several smaller paid apps do this one job.
• Hammerspoon (free, open source) can do anything here if you write the Lua yourself. Hammerdeck grew out of years of Hammerspoon scripts, and exists for the people who would rather not.

Hammerdeck, Rectangle or Moom compares them in more detail.