fix(tui): do not invoke loop recursively for pad()

fixes #21610

src/nvim/tui/tui.c

@@ -1626,7 +1626,7 @@ static void pad(void *ctx, size_t delay, int scale FUNC_ATTR_UNUSED, int force)
   }
 
   flush_buf(tui);
-  loop_uv_run(tui->loop, (int64_t)(delay / 10), false);
+  uv_sleep((unsigned int)(delay/10));
 }
 
 static void unibi_set_if_empty(unibi_term *ut, enum unibi_string str, const char *val)

test/functional/terminal/tui_spec.lua

@@ -1469,6 +1469,33 @@ describe('TUI', function()
                                                                                  |
       {3:-- TERMINAL --}                                                             |]]))
   end)
+
+  it('visual bell (padding) does not crash #21610', function()
+    feed_data ':set visualbell\n'
+    screen:expect{grid=[[
+      {1: }                                                 |
+      {4:~                                                 }|
+      {4:~                                                 }|
+      {4:~                                                 }|
+      {5:[No Name]                                         }|
+      :set visualbell                                   |
+      {3:-- TERMINAL --}                                    |
+    ]]}
+
+    -- move left is enough to invoke the bell
+    feed_data 'h'
+    -- visual change to show we process events after this
+    feed_data 'i'
+    screen:expect{grid=[[
+      {1: }                                                 |
+      {4:~                                                 }|
+      {4:~                                                 }|
+      {4:~                                                 }|
+      {5:[No Name]                                         }|
+      {3:-- INSERT --}                                      |
+      {3:-- TERMINAL --}                                    |
+    ]]}
+  end)
 end)
 
 describe('TUI', function()