on [<modes>]window_beforekill [<serial#>] [-|^]<match> { <action> }

This hook is triggered just before a window is killed. It is permissible to query any data you want about the window, but it is NOT PERMISSIBLE to try to change anything about the window, particularly its visibility. If you try to change the window, you may crash the client. Use /DEFER to be on the safe side.

This hook represents your last chance to save data about a window before everything about that window's history is permanently erased.