fix(showcmd): clear the rest properly (#28420)

src/nvim/grid.c

@@ -436,9 +436,8 @@ int grid_line_puts(int col, const char *text, int textlen, int attr)
                     ? utfc_ptr2schar_len(ptr, (int)((text + len) - ptr), &firstc)
                     : utfc_ptr2schar(ptr, &firstc);
     int mbyte_cells = utf_char2cells(firstc);
-    if (mbyte_cells > 2) {
+    if (mbyte_cells > 2 || schar == 0) {
       mbyte_cells = 1;
-
       schar = schar_from_char(0xFFFD);
     }
 

src/nvim/normal.c

@@ -2043,8 +2043,7 @@ void pop_showcmd(void)
 
 static void display_showcmd(void)
 {
-  int len = vim_strsize(showcmd_buf);
+  showcmd_is_clear = (showcmd_buf[0] == NUL);
-  showcmd_is_clear = (len == 0);
 
   if (*p_sloc == 's') {
     if (showcmd_is_clear) {
@@ -2069,7 +2068,7 @@ static void display_showcmd(void)
   if (ui_has(kUIMessages)) {
     MAXSIZE_TEMP_ARRAY(content, 1);
     MAXSIZE_TEMP_ARRAY(chunk, 2);
-    if (len > 0) {
+    if (!showcmd_is_clear) {
       // placeholder for future highlight support
       ADD_C(chunk, INTEGER_OBJ(0));
       ADD_C(chunk, CSTR_AS_OBJ(showcmd_buf));
@@ -2086,8 +2085,9 @@ static void display_showcmd(void)
   int showcmd_row = Rows - 1;
   grid_line_start(&msg_grid_adj, showcmd_row);
 
+  int len = 0;
   if (!showcmd_is_clear) {
-    grid_line_puts(sc_col, showcmd_buf, -1, HL_ATTR(HLF_MSG));
+    len = grid_line_puts(sc_col, showcmd_buf, -1, HL_ATTR(HLF_MSG));
   }
 
   // clear the rest of an old message by outputting up to SHOWCMD_COLS spaces

test/functional/editor/mode_normal_spec.lua

@@ -1,6 +1,7 @@
 -- Normal mode tests.
 
 local t = require('test.functional.testutil')()
+local Screen = require('test.functional.ui.screen')
 local clear = t.clear
 local feed = t.feed
 local fn = t.fn
@@ -19,4 +20,23 @@ describe('Normal mode', function()
     feed('k')
     eq(pos, fn.getcurpos())
   end)
+
+  it('&showcmd does not crash with :startinsert #28419', function()
+    local screen = Screen.new(60, 17)
+    screen:attach()
+    fn.termopen(
+      { t.nvim_prog, '--clean', '--cmd', 'startinsert' },
+      { env = { VIMRUNTIME = os.getenv('VIMRUNTIME') } }
+    )
+    screen:expect({
+      grid = [[
+        ^                                                            |
+        ~                                                           |*13
+        [No Name]                                 0,1            All|
+        -- INSERT --                                                |
+                                                                    |
+      ]],
+      attr_ids = {},
+    })
+  end)
 end)