window_close_confirmation

Whether to display a confirmation prompt when the window is closed by the windowing environment, either because the user closed it with the window decorations, or instructed their window manager to close it.

Set this to "NeverPrompt" if you don't like confirming closing windows every time.

return {
  window_close_confirmation = "AlwaysPrompt",
}

See also skip_close_confirmation_for_processes_named.

Note that this window_close_confirmation option doesn't apply to the default CTRL-SHIFT-W or CMD-w key assignments; if you want to change prompts for those, you will need to override the key shortcut as shown in the CloseCurrentTab documentation.