vim-patch:9.1.0118: Use different restoration strategy in win_splitmove

Problem:  saving and restoring all frames to split-move is overkill now
          that WinNewPre is not fired when split-moving.
Solution: defer the flattening of frames until win_split_ins begins
          reorganising them, and attempt to restore the layout by
          undoing our changes. (Sean Dewar)

704966c254

Adjust winframe_restore to account for Nvim's horizontal separators when the
global statusline is in use. Add a test.

test/functional/ui/statusline_spec.lua

@@ -11,6 +11,7 @@ local exec = helpers.exec
 local exec_lua = helpers.exec_lua
 local eval = helpers.eval
 local sleep = vim.uv.sleep
+local pcall_err = helpers.pcall_err
 
 local mousemodels = { 'extend', 'popup', 'popup_setpos' }
 
@@ -474,6 +475,25 @@ describe('global statusline', function()
                                                                   |
     ]])
   end)
+
+  it('horizontal separators unchanged when failing to split-move window', function()
+    exec([[
+      botright split
+      let &winwidth = &columns
+      let &winminwidth = &columns
+    ]])
+    eq('Vim(wincmd):E36: Not enough room', pcall_err(command, 'wincmd L'))
+    command('mode')
+    screen:expect([[
+                                                                  |
+      {1:~                                                           }|*5
+      ────────────────────────────────────────────────────────────|
+      ^                                                            |
+      {1:~                                                           }|*6
+      {2:[No Name]                                 0,0-1          All}|
+                                                                  |
+    ]])
+  end)
 end)
 
 it('statusline does not crash if it has Arabic characters #19447', function()