fix(ui): set 'cmdheight' to zero for all open tabpages

Problem:  Enabling ext_messages claims to set 'cmdheight' to zero, but
only does so for the current tabpage.
Solution: Set stored 'cmdheight' value to zero for all tabpages.

src/nvim/ui.c

@@ -218,10 +218,13 @@ void ui_refresh(void)
     if (i < kUIGlobalCount) {
       ext_widgets[i] |= ui_cb_ext[i];
     }
-    // Set 'cmdheight' to zero when ext_messages becomes active.
+    // Set 'cmdheight' to zero when ext_messages becomes active for all tabpages.
     if (i == kUIMessages && !ui_ext[i] && ext_widgets[i]) {
       set_option_value(kOptCmdheight, NUMBER_OPTVAL(0), 0);
       command_height();
+      FOR_ALL_TABS(tp) {
+        tp->tp_ch_used = 0;
+      }
     }
     ui_ext[i] = ext_widgets[i];
     if (i < kUIGlobalCount) {

test/functional/lua/ui_event_spec.lua

@@ -163,6 +163,13 @@ describe('vim.ui_attach', function()
     screen:attach()
     eq(1, n.api.nvim_get_option_value('cmdheight', {}))
   end)
+
+  it("ui_refresh() sets 'cmdheight' for all open tabpages with ext_messages", function()
+    exec_lua('vim.cmd.tabnew()')
+    exec_lua('vim.ui_attach(ns, { ext_messages = true }, on_event)')
+    exec_lua('vim.cmd.tabnext()')
+    eq(0, n.api.nvim_get_option_value('cmdheight', {}))
+  end)
 end)
 
 describe('vim.ui_attach', function()