Spawn tmux fg job in bg window when height is null

When the requested height for a tmux split is null and that the spawned
job will be able to call back through the GUI subsystem, spawn the
foreground job in a new background window, just as if it were a
background job
Note that setting the height to 0 works even when the GUI callback
mechanism is not available since "split-window -l 0" creates a two lines
split (tested with tmux 2.5)

Author: shym

autoload/dispatch/tmux.vim

@@ -49,7 +49,7 @@ function! dispatch#tmux#make(request) abort
 
   let title = shellescape(get(a:request, 'title', get(a:request, 'compiler', 'make')))
   let height = get(g:, 'dispatch_tmux_height', get(g:, 'dispatch_quickfix_height', 10))
-  if get(a:request, 'background', 0)
+  if get(a:request, 'background', 0) || (height <= 0 && has('clientserver') && !empty(v:servername))
     let cmd = 'new-window -d -n '.title
   elseif has('gui_running') || empty($TMUX) || (!empty(''.session) && session !=# system('tmux display-message -p "#S"')[0:-2])
     let cmd = 'new-window -n '.title