Fix regression in closing project diff (#43964)

Follow-up to #43586--when the diff is not split, just render the primary
editor. Otherwise the child `Pane` intercepts `CloseActiveItem`. (This
is still a bug for the actual split diff, but just fixing the
user-visible regression for now.)

Release Notes:

- N/A
a675eb1667Cole Miller committed on 12/2/2025, 2:32:36 AM· committed by GitHubparentb27ad98
1 file changedLine totals unavailable