Small split-move related improvements

Problem: small improvements can be made to split-move related functions.

Solution: apply them:

- 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 vim#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 vim#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.

src/evalwindow.c

@@ -1001,8 +1001,8 @@ f_win_splitmove(typval_T *argvars, typval_T *rettv)
  size = (int)dict_get_number(d, "size");
  }
 
- // Check if we can split the target before we bother switching windows.
- if (text_or_buf_locked() || check_split_disallowed(targetwin) == FAIL)
+ // Check if we're allowed to continue before we bother switching windows.
+ if (text_or_buf_locked() || check_split_disallowed(wp) == FAIL)
  return;
 
  if (curwin != targetwin)

src/testdir/test_window_cmd.vim

@@ -276,6 +276,20 @@ func Test_window_split_edit_bufnr()
  %bw!
 endfunc
 
+func s:win_layout_info() abort
+ return #{
+ \ layout: winlayout(),
+ \ pos_sizes: range(1, winnr('$'))
+ \  ->map({_, nr -> win_getid(nr)->getwininfo()[0]})
+ \  ->map({_, wininfo -> #{id: wininfo.winid,
+ \  row: wininfo.winrow,
+ \  col: wininfo.wincol,
+ \  width: wininfo.width,
+ \  height: wininfo.height}})
+ \  ->sort({a, b -> a.id - b.id})
+ \ }
+endfunc
+
 func Test_window_split_no_room()
  " N horizontal windows need >= 2*N + 1 lines:
  " - 1 line + 1 status line in each window
@@ -292,12 +306,10 @@ func Test_window_split_no_room()
 
  botright vsplit
  wincmd |
- let layout = winlayout()
- let restcmd = winrestcmd()
+ let info = s:win_layout_info()
  call assert_fails('wincmd J', 'E36:')
  call assert_fails('wincmd K', 'E36:')
- call assert_equal(layout, winlayout())
- call assert_equal(restcmd, winrestcmd())
+ call assert_equal(info, s:win_layout_info())
  only
 
  " N vertical windows need >= 2*(N - 1) + 1 columns:
@@ -314,30 +326,28 @@ func Test_window_split_no_room()
 
  split
  wincmd |
- let layout = winlayout()
- let restcmd = winrestcmd()
+ let info = s:win_layout_info()
  call assert_fails('wincmd H', 'E36:')
  call assert_fails('wincmd L', 'E36:')
- call assert_equal(layout, winlayout())
- call assert_equal(restcmd, winrestcmd())
+ call assert_equal(info, s:win_layout_info())
 
  " Check that the last statusline isn't lost.
  " Set its window's width to 2 for the test.
  wincmd j
  set laststatus=0 winminwidth=0
  vertical resize 2
+ " Update expected positions/sizes after the resize. Layout is unchanged.
+ let info.pos_sizes = s:win_layout_info().pos_sizes
  set winminwidth&
  call setwinvar(winnr('k'), '&statusline', '@#')
  let last_stl_row = win_screenpos(0)[0] - 1
  redraw
  call assert_equal('@#|', GetScreenStr(last_stl_row))
  call assert_equal('~ |', GetScreenStr(&lines - &cmdheight))
 
- let restcmd = winrestcmd()
  call assert_fails('wincmd H', 'E36:')
  call assert_fails('wincmd L', 'E36:')
- call assert_equal(layout, winlayout())
- call assert_equal(restcmd, winrestcmd())
+ call assert_equal(info, s:win_layout_info())
  call setwinvar(winnr('k'), '&statusline', '=-')
  redraw
  call assert_equal('=-|', GetScreenStr(last_stl_row))
@@ -1175,6 +1185,7 @@ func Test_win_splitmove()
  augroup WinSplitMove
  au!
  au WinEnter * ++once let s:triggered = v:true
+ \| call assert_fails('call win_splitmove(winnr(), winnr("$"))', 'E242:')
  \| call assert_fails('call win_splitmove(winnr("$"), winnr())', 'E242:')
  augroup END
  quit
@@ -1185,7 +1196,7 @@ func Test_win_splitmove()
  augroup WinSplitMove
  au!
  au BufHidden * ++once let s:triggered = v:true
- \| call assert_fails('call win_splitmove(winnr("#"), winnr())', 'E1159:')
+ \| call assert_fails('call win_splitmove(winnr(), winnr("#"))', 'E1159:')
  augroup END
  hide
  call assert_equal(v:true, s:triggered)
@@ -2211,13 +2222,11 @@ func Test_autocmd_window_force_room()
  au BufEnter * ++once let s:triggered = v:true
  \| call assert_equal('autocmd', win_gettype())
  augroup END
- let layout = winlayout()
- let restcmd = winrestcmd()
+ let info = s:win_layout_info()
  " bufload opening the autocommand window shouldn't give E36.
  call bufload('unload me')
  call assert_equal(v:true, s:triggered)
- call assert_equal(winlayout(), layout)
- call assert_equal(winrestcmd(), restcmd)
+ call assert_equal(info, s:win_layout_info())
 
  unlet! s:triggered
  au! AucmdWinForceRoom

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);
 }
 
 /*