fix(window): don't add a hsep when out of room if global stl is off

Problem: a horizontal separator may be added to a window that doesn't need one
if there is no room when moving a different window.

Solution: only restore a hsep in winframe_restore when the global statusline is
enabled.

(cherry picked from commit bf5f7c1591)

src/nvim/window.c

@@ -3359,7 +3359,7 @@ void winframe_restore(win_T *wp, int dir, frame_T *unflat_altfr)
   if (frp->fr_parent->fr_layout == FR_COL && frp->fr_prev != NULL) {
     if (global_stl_height() == 0 && wp->w_status_height == 0) {
       frame_add_statusline(frp->fr_prev);
-    } else if (wp->w_hsep_height == 0) {
+    } else if (global_stl_height() > 0 && wp->w_hsep_height == 0) {
       frame_add_hsep(frp->fr_prev);
     }
   }

test/functional/ui/statusline_spec.lua

@@ -504,6 +504,20 @@ describe('global statusline', function()
       {3:[No Name]                                 0,0-1          All}|
                                                                   |
     ]])
+
+    -- Shouldn't gain a hsep if the global statusline is turned off.
+    command('set laststatus=2')
+    eq('Vim(wincmd):E36: Not enough room', pcall_err(command, 'wincmd L'))
+    command('mode')
+    screen:expect([[
+                                                                  |
+      {1:~                                                           }|*5
+      {2:[No Name]                                 0,0-1          All}|
+      ^                                                            |
+      {1:~                                                           }|*6
+      {3:[No Name]                                 0,0-1          All}|
+                                                                  |
+    ]])
   end)
 end)