patch 9.1.0171: Small split-move related improvements

Problem:  small improvements can be made to split-move related
          functions.
Solution: apply them (Sean Dewar):

- Improve some doc comments (frame_flatten should still work for non-current
  tabpages, despite the topframe check, which looks benign, though I'm unsure if
  it's still needed; see #2467).

- f_win_splitmove should check_split_disallowed on wp, not targetwin, as that's
  what win_splitmove checks (though it's probably unnecessary to check
  b_locked_split at all; see #14109, which I hope to get around to finishing at
  some point).

- Make winframe_restore restore window positions for the altframe, which
  winframe_remove changes. This doesn't affect the prior behaviour, as we called
  win_comp_pos after, but as win_comp_pos only works for curtab, and
  winframe_remove supports non-current tabpages, we should undo it. Regardless,
  this should mean we don't need win_comp_pos anymore; adjust tests to check
  that window positions remain unchanged.

  I'm not sure win_comp_pos is needed after last_status anyway if it doesn't
  steal rows from another frame to make room for a new statusline, which
  shouldn't be the case after winframe_remove? To be safe, I'll leave it as is.

closes: #14185

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/window.c

@@ -896,8 +896,8 @@ cmd_with_count(
 }
 
 /*
- * If "split_disallowed" is set for "wp", give an error and return FAIL.
- * Otherwise return OK.
+ * If "split_disallowed" is set, or "wp"'s buffer is closing, give an error and
+ * return FAIL.  Otherwise return OK.
  */
     int
 check_split_disallowed(win_T *wp)
@@ -1980,7 +1980,6 @@ win_splitmove(win_T *wp, int size, int flags)
 	// existing window, so just undo winframe_remove.
 	winframe_restore(wp, dir, unflat_altfr);
 	win_append(wp->w_prev, wp);
-	(void)win_comp_pos();   // recompute window positions
 	return FAIL;
     }
 
@@ -3607,7 +3606,6 @@ winframe_remove(
  * Flatten "frp" into its parent frame if it's the only child, also merging its
  * list with the grandparent if they share the same layout.
  * Frees "frp" if flattened; also "frp->fr_parent" if it has the same layout.
- * "frp" must be valid in the current tabpage.
  */
     static void
 frame_flatten(frame_T *frp)
@@ -3660,13 +3658,16 @@ frame_flatten(frame_T *frp)
 
 /*
  * Undo changes from a prior call to winframe_remove, also restoring lost
- * vertical separators and statuslines.
+ * vertical separators and statuslines, and changed window positions for
+ * windows within "unflat_altfr".
  * Caller must ensure no other changes were made to the layout or window sizes!
  */
     static void
 winframe_restore(win_T *wp, int dir, frame_T *unflat_altfr)
 {
     frame_T	*frp = wp->w_frame;
+    int		row = wp->w_winrow;
+    int		col = wp->w_wincol;
 
     // Put "wp"'s frame back where it was.
     if (frp->fr_prev != NULL)
@@ -3684,17 +3685,25 @@ winframe_restore(win_T *wp, int dir, frame_T *unflat_altfr)
 	    && frp->fr_parent->fr_layout == FR_COL && frp->fr_prev != NULL)
 	frame_add_statusline(frp->fr_prev);
 
-    // Restore the lost room that was redistributed to the altframe.
+    // Restore the lost room that was redistributed to the altframe.  Also
+    // adjusts window sizes to fit restored statuslines/separators, if needed.
     if (dir == 'v')
     {
 	frame_new_height(unflat_altfr, unflat_altfr->fr_height - frp->fr_height,
 		unflat_altfr == frp->fr_next, FALSE);
+	row += frp->fr_height;
     }
     else if (dir == 'h')
     {
 	frame_new_width(unflat_altfr, unflat_altfr->fr_width - frp->fr_width,
 		unflat_altfr == frp->fr_next, FALSE);
+	col += frp->fr_width;
     }
+
+    // If rows/columns went to a window below/right, its positions need to be
+    // restored.  Can only be done after the sizes have been updated.
+    if (unflat_altfr == frp->fr_next)
+	frame_comp_pos(unflat_altfr, &row, &col);
 }
 
 /*