vim-patch:9.1.0892: the max value of 'cmdheight' is limited by other tabpages (#31378)

Problem:  the max value of 'cmdheight' is limited by other tabpages
Solution: Limit the maximum value of 'cmdheight' to the current tabpage only.
          (Milly)

The Help says that cmdheight is local to the tab page, but says nothing
about the maximum value depending on the state of all tab pages. Users
may wonder why they can't increase cmdheight when there are still rows
available on the current tab page. This PR changes the behavior of
cmdheight so that its maximum value depends only on the state of the
current tab page.

Also, since magic numbers were embedded in various places with the
minimum value of cmdheight being 1, we defined a constant to make it
easier to understand.

closes: vim/vim#16131

https://github.com/vim/vim/commit/2cddf0e85a7f8304476397e1c51dcd0e41835ac3

Cherry-pick Test_cmdheight_not_changed() from patch 9.0.0187.

Co-authored-by: Milly <milly.ca@gmail.com>

3 files changed, 22 insertions, 5 deletions

diff --git a/src/nvim/drawscreen.c b/src/nvim/drawscreen.c
index e645d1bbea..835fdcf7d0 100644
--- a/src/nvim/drawscreen.c
+++ b/src/nvim/drawscreen.c
@@ -396,7 +396,7 @@ void check_screensize(void)
 {
   // Limit Rows and Columns to avoid an overflow in Rows * Columns.
   // need room for one window and command line
-  Rows = MIN(MAX(Rows, min_rows()), 1000);
+  Rows = MIN(MAX(Rows, min_rows_for_all_tabpages()), 1000);
   Columns = MIN(MAX(Columns, MIN_COLUMNS), 10000);
 }
 
diff --git a/src/nvim/option.c b/src/nvim/option.c
index 27b80c0ac8..6da9635479 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -2764,10 +2764,11 @@ static const char *check_num_option_bounds(OptIndex opt_idx, OptInt *newval, cha
 
   switch (opt_idx) {
   case kOptLines:
-    if (*newval < min_rows() && full_screen) {
-      vim_snprintf(errbuf, errbuflen, _("E593: Need at least %d lines"), min_rows());
+    if (*newval < min_rows_for_all_tabpages() && full_screen) {
+      vim_snprintf(errbuf, errbuflen, _("E593: Need at least %d lines"),
+                   min_rows_for_all_tabpages());
       errmsg = errbuf;
-      *newval = min_rows();
+      *newval = min_rows_for_all_tabpages();
     }
     // True max size is defined by check_screensize().
     *newval = MIN(*newval, INT_MAX);
diff --git a/src/nvim/window.c b/src/nvim/window.c
index ac4c5a8e4a..d92b2ab601 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -6189,7 +6189,7 @@ const char *did_set_winminheight(optset_T *args FUNC_ATTR_UNUSED)
   // loop until there is a 'winminheight' that is possible
   while (p_wmh > 0) {
     const int room = Rows - (int)p_ch;
-    const int needed = min_rows();
+    const int needed = min_rows_for_all_tabpages();
     if (room >= needed) {
       break;
     }
@@ -7072,6 +7072,22 @@ int min_rows(void)
     return MIN_LINES;
   }
 
+  int total = frame_minheight(curtab->tp_topframe, NULL);
+  total += tabline_height() + global_stl_height();
+  if (p_ch > 0) {
+    total += 1;           // count the room for the command line
+  }
+  return total;
+}
+
+/// Return the minimal number of rows that is needed on the screen to display
+/// the current number of windows for all tab pages.
+int min_rows_for_all_tabpages(void)
+{
+  if (firstwin == NULL) {       // not initialized yet
+    return MIN_LINES;
+  }
+
   int total = 0;
   FOR_ALL_TABS(tp) {
     int n = frame_minheight(tp->tp_topframe, NULL);