Add -B to new-pane/split-window to block until the command exits, like

run-shell -b.

tmux.1

@@ -3380,7 +3380,7 @@ but a different format may be specified with
 .Fl F .
 .Tg newp
 .It Xo Ic new\-pane
-.Op Fl bdefhIkPvZ
+.Op Fl bBdefhIkPvZ
 .Op Fl c Ar start\-directory
 .Op Fl e Ar environment
 .Op Fl F Ar format
@@ -3456,6 +3456,17 @@ but also sets the
 option for this pane to
 .Ar message .
 .Pp
+.Fl B
+blocks until
+.Ar shell\-command
+exits, then returns its exit status.
+For example:
+.Bd -literal -offset indent
+$ tmux new-pane -B 'vi afile'
+$ echo $?
+0
+.Ed
+.Pp
 .Fl E ,
 or an empty
 .Ar shell\-command ,