fix(tabpage): check if ROWS_AVAIL changed for resize (#19620)

N/A patches for version.c:

vim-patch:9.0.0135: comment about tabpage line above the wrong code

Problem:    Comment about tabpage line above the wrong code.
Solution:   Move the comment. (closes vim/vim#10836)
https://github.com/vim/vim/commit/0b0ccbbfb014e1c9682b86a7a41ff2837b0b8047

3 files changed, 55 insertions, 16 deletions

diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index aeba3acbc3..3ef5ea7e02 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -947,16 +947,15 @@ struct diffblock_S {
 typedef struct tabpage_S tabpage_T;
 struct tabpage_S {
   handle_T handle;
-  tabpage_T *tp_next;         ///< next tabpage or NULL
-  frame_T *tp_topframe;     ///< topframe for the windows
-  win_T *tp_curwin;       ///< current window in this Tab page
-  win_T *tp_prevwin;      ///< previous window in this Tab page
-  win_T *tp_firstwin;     ///< first window in this Tab page
-  win_T *tp_lastwin;      ///< last window in this Tab page
-  long tp_old_Rows;                 ///< Rows when Tab page was left
-  long tp_old_Columns;              ///< Columns when Tab page was left
-  long tp_ch_used;                  ///< value of 'cmdheight' when frame size
-                                    ///< was set
+  tabpage_T *tp_next;      ///< next tabpage or NULL
+  frame_T *tp_topframe;    ///< topframe for the windows
+  win_T *tp_curwin;        ///< current window in this Tab page
+  win_T *tp_prevwin;       ///< previous window in this Tab page
+  win_T *tp_firstwin;      ///< first window in this Tab page
+  win_T *tp_lastwin;       ///< last window in this Tab page
+  long tp_old_Rows_avail;  ///< ROWS_AVAIL when Tab page was left
+  long tp_old_Columns;     ///< Columns when Tab page was left
+  long tp_ch_used;         ///< value of 'cmdheight' when frame size was set
 
   diff_T *tp_first_diff;
   buf_T *(tp_diffbuf[DB_COUNT]);
diff --git a/src/nvim/window.c b/src/nvim/window.c
index e820be626c..c7f038850e 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -4293,7 +4293,7 @@ static int leave_tabpage(buf_T *new_curbuf, bool trigger_leave_autocmds)
   tp->tp_prevwin = prevwin;
   tp->tp_firstwin = firstwin;
   tp->tp_lastwin = lastwin;
-  tp->tp_old_Rows = Rows;
+  tp->tp_old_Rows_avail = ROWS_AVAIL;
   tp->tp_old_Columns = Columns;
   firstwin = NULL;
   lastwin = NULL;
@@ -4333,10 +4333,7 @@ static void enter_tabpage(tabpage_T *tp, buf_T *old_curbuf, bool trigger_enter_a
   const int row = win_comp_pos();  // recompute w_winrow for all windows
   diff_need_scrollbind = true;
 
-  // The tabpage line may have appeared or disappeared, may need to resize
-  // the frames for that.  When the Vim window was resized need to update
-  // frame sizes too.  Use the stored value of p_ch, so that it can be
-  // different for each tab page.
+  // Use the stored value of p_ch, so that it can be different for each tab page.
   if (p_ch != curtab->tp_ch_used) {
     clear_cmdline = true;
   }
@@ -4349,7 +4346,9 @@ static void enter_tabpage(tabpage_T *tp, buf_T *old_curbuf, bool trigger_enter_a
     clear_cmdline = true;
   }
 
-  if (curtab->tp_old_Rows != Rows || (old_off != firstwin->w_winrow)) {
+  // The tabpage line may have appeared or disappeared, may need to resize the frames for that.
+  // When the Vim window was resized or ROWS_AVAIL changed need to update frame sizes too.
+  if (curtab->tp_old_Rows_avail != ROWS_AVAIL || (old_off != firstwin->w_winrow)) {
     win_new_screen_rows();
   }
   if (curtab->tp_old_Columns != Columns && starting == 0) {
diff --git a/test/functional/editor/tabpage_spec.lua b/test/functional/editor/tabpage_spec.lua
index 3b2c1db350..7dd0b9f154 100644
--- a/test/functional/editor/tabpage_spec.lua
+++ b/test/functional/editor/tabpage_spec.lua
@@ -1,4 +1,5 @@
 local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
 
 local clear = helpers.clear
 local command = helpers.command
@@ -53,6 +54,46 @@ describe('tabpage', function()
     neq(999, eval('g:win_closed'))
   end)
 
+  it('switching tabpage after setting laststatus=3 #19591', function()
+    local screen = Screen.new(40, 8)
+    screen:set_default_attr_ids({
+      [0] = {bold = true, foreground = Screen.colors.Blue},
+      [1] = {bold = true, reverse = true},  -- StatusLine
+      [2] = {reverse = true},  -- StatusLineNC, TabLineFill
+      [3] = {bold = true}, -- TabLineSel
+      [4] = {background = Screen.colors.LightGrey, underline = true},  -- TabLine
+      [5] = {bold = true, foreground = Screen.colors.Magenta},
+    })
+    screen:attach()
+
+    command('tabnew')
+    command('tabprev')
+    command('set laststatus=3')
+    command('tabnext')
+    feed('<C-G>')
+    screen:expect([[
+      {4: [No Name] }{3: [No Name] }{2:                 }{4:X}|
+      ^                                        |
+      {0:~                                       }|
+      {0:~                                       }|
+      {0:~                                       }|
+      {0:~                                       }|
+      {1:[No Name]                               }|
+      "[No Name]" --No lines in buffer--      |
+    ]])
+    command('vnew')
+    screen:expect([[
+      {4: [No Name] }{3: }{5:2}{3: [No Name] }{2:               }{4:X}|
+      ^                    │                   |
+      {0:~                   }│{0:~                  }|
+      {0:~                   }│{0:~                  }|
+      {0:~                   }│{0:~                  }|
+      {0:~                   }│{0:~                  }|
+      {1:[No Name]                               }|
+      "[No Name]" --No lines in buffer--      |
+    ]])
+  end)
+
   it(":tabmove handles modifiers and addr", function()
     command('tabnew | tabnew | tabnew')
     eq(4, funcs.nvim_tabpage_get_number(0))