fix(ui): superfluous showmode / excessive grid_cursor_goto #29089

Problem:  Unsetting global variables earlier in #28578 to avoid
          recursiveness, caused superfluous or even unlimited
          showmode().
Solution: Partly revert #28578 so that the globals are unset at the end
          of showmode(), and avoid recursiveness for ext UI by adding a
          recursive function guard to each generated UI call that may
          call a Lua callback.

src/nvim/drawscreen.c

@@ -931,13 +931,7 @@ int showmode(void)
     msg_ext_clear(true);
   }
 
-  // Don't make non-flushed message part of the showmode and reset global
+  // Don't make non-flushed message part of the showmode.
-  // variables before flushing to to avoid recursiveness.
-  bool draw_mode = redraw_mode;
-  bool clear_cmd = clear_cmdline;
-  redraw_cmdline = false;
-  redraw_mode = false;
-  clear_cmdline = false;
   msg_ext_ui_flush();
 
   msg_grid_validate();
@@ -960,8 +954,8 @@ int showmode(void)
     msg_check_for_delay(false);
 
     // if the cmdline is more than one line high, erase top lines
-    bool need_clear = clear_cmd;
+    bool need_clear = clear_cmdline;
-    if (clear_cmd && cmdline_row < Rows - 1) {
+    if (clear_cmdline && cmdline_row < Rows - 1) {
       msg_clr_cmdline();  // will reset clear_cmdline
     }
 
@@ -1083,7 +1077,7 @@ int showmode(void)
     }
 
     mode_displayed = true;
-    if (need_clear || clear_cmd || draw_mode) {
+    if (need_clear || clear_cmdline || redraw_mode) {
       msg_clr_eos();
     }
     msg_didout = false;                 // overwrite this message
@@ -1092,10 +1086,10 @@ int showmode(void)
     msg_no_more = false;
     lines_left = save_lines_left;
     need_wait_return = nwr_save;        // never ask for hit-return for this
-  } else if (clear_cmd && msg_silent == 0) {
+  } else if (clear_cmdline && msg_silent == 0) {
     // Clear the whole command line.  Will reset "clear_cmdline".
     msg_clr_cmdline();
-  } else if (draw_mode) {
+  } else if (redraw_mode) {
     msg_pos_mode();
     msg_clr_eos();
   }
@@ -1118,6 +1112,10 @@ int showmode(void)
     grid_line_flush();
   }
 
+  redraw_cmdline = false;
+  redraw_mode = false;
+  clear_cmdline = false;
+
   return length;
 }
 

src/nvim/ex_getln.c

@@ -3455,11 +3455,9 @@ void cmdline_screen_cleared(void)
 /// called by ui_flush, do what redraws necessary to keep cmdline updated.
 void cmdline_ui_flush(void)
 {
-  static bool flushing = false;
+  if (!ui_has(kUICmdline)) {
-  if (!ui_has(kUICmdline) || flushing) {
     return;
   }
-  flushing = true;
   int level = ccline.level;
   CmdlineInfo *line = &ccline;
   while (level > 0 && line) {
@@ -3474,7 +3472,6 @@ void cmdline_ui_flush(void)
     }
     line = line->prev_ccline;
   }
-  flushing = false;
 }
 
 // Put a character on the command line.  Shifts the following text to the

src/nvim/generators/gen_api_ui_events.lua

@@ -128,8 +128,16 @@ for i = 1, #events do
     write_signature(call_output, ev, '')
     call_output:write('\n{\n')
     if ev.remote_only then
+      -- Lua callbacks may emit other events or the same event again. Avoid the latter
+      -- by adding a recursion guard to each generated function that may call a Lua callback.
+      call_output:write('  static bool entered = false;\n')
+      call_output:write('  if (entered) {\n')
+      call_output:write('    return;\n')
+      call_output:write('  }\n')
+      call_output:write('  entered = true;\n')
       write_arglist(call_output, ev)
       call_output:write('  ui_call_event("' .. ev.name .. '", ' .. args .. ');\n')
+      call_output:write('  entered = false;\n')
     elseif ev.compositor_impl then
       call_output:write('  ui_comp_' .. ev.name)
       write_signature(call_output, ev, '', true)

test/functional/ui/messages_spec.lua

@@ -1081,6 +1081,22 @@ stack traceback:
       },
     })
   end)
+
+  it('does not do showmode unnecessarily #29086', function()
+    local screen_showmode = screen._handle_msg_showmode
+    local showmode = 0
+    screen._handle_msg_showmode = function(...)
+      screen_showmode(...)
+      showmode = showmode + 1
+    end
+    screen:expect({
+      grid = [[
+        ^                         |
+        {1:~                        }|*4
+      ]],
+    })
+    eq(showmode, 1)
+  end)
 end)
 
 describe('ui/builtin messages', function()