fix(grid): handle clearing half a double-width char (#27023)

src/nvim/grid.c

@@ -655,6 +655,7 @@ void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int col, int endcol
   // two-cell character in the same grid, truncate that into a '>'.
   if (col > 0 && grid->chars[off_to + (size_t)col] == 0) {
     linebuf_char[col - 1] = schar_from_ascii('>');
+    linebuf_attr[col - 1] = grid->attrs[off_to + (size_t)col - 1];
     col--;
   }
 
@@ -676,7 +677,7 @@ void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int col, int endcol
     }
   }
 
-  redraw_next = grid_char_needs_redraw(grid, col, (size_t)col + off_to, endcol - col);
+  redraw_next = grid_char_needs_redraw(grid, col, off_to + (size_t)col, endcol - col);
 
   int start_dirty = -1;
   int end_dirty = 0;
@@ -688,7 +689,7 @@ void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int col, int endcol
       char_cells = 2;
     }
     bool redraw_this = redraw_next;  // Does character need redraw?
-    size_t off = (size_t)col + off_to;
+    size_t off = off_to + (size_t)col;
     redraw_next = grid_char_needs_redraw(grid, col + char_cells,
                                          off + (size_t)char_cells,
                                          endcol - col - char_cells);
@@ -732,16 +733,22 @@ void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int col, int endcol
   if (clear_next) {
     // Clear the second half of a double-wide character of which the left
     // half was overwritten with a single-wide character.
-    grid->chars[(size_t)col + off_to] = schar_from_ascii(' ');
+    grid->chars[off_to + (size_t)col] = schar_from_ascii(' ');
     end_dirty++;
   }
 
+  // When clearing the left half of a double-wide char also clear the right half.
+  if (off_to + (size_t)clear_width < max_off_to
+      && grid->chars[off_to + (size_t)clear_width] == 0) {
+    clear_width++;
+  }
+
   int clear_dirty_start = -1, clear_end = -1;
   // blank out the rest of the line
   // TODO(bfredl): we could cache winline widths
   col = clear_start;
   while (col < clear_width) {
-    size_t off = (size_t)col + off_to;
+    size_t off = off_to + (size_t)col;
     if (grid->chars[off] != schar_from_ascii(' ')
         || grid->attrs[off] != bg_attr
         || rdb_flags & RDB_NODELTA) {

test/functional/ui/multibyte_spec.lua

@@ -22,6 +22,8 @@ describe('multibyte rendering', function()
       [3] = { background = Screen.colors.LightMagenta },
       [4] = { bold = true },
       [5] = { foreground = Screen.colors.Blue },
+      [6] = { reverse = true, bold = true },
+      [7] = { reverse = true },
     })
   end)
 
@@ -84,7 +86,7 @@ describe('multibyte rendering', function()
       {4:-- INSERT --}                                                |
     ]])
 
-    -- check double-with char is temporarily hidden when overlapped
+    -- check double-width char is temporarily hidden when overlapped
     fn.complete(4, { 'xx', 'yy' })
     screen:expect([[
       ab xx^                                                       |
@@ -104,6 +106,34 @@ describe('multibyte rendering', function()
     ]])
   end)
 
+  it('no stray chars when splitting left of window with double-width chars', function()
+    api.nvim_buf_set_lines(0, 0, -1, true, {
+      ('口'):rep(16),
+      'a' .. ('口'):rep(16),
+      'aa' .. ('口'):rep(16),
+      'aaa' .. ('口'):rep(16),
+      'aaaa' .. ('口'):rep(16),
+    })
+    screen:expect([[
+      ^口口口口口口口口口口口口口口口口                            |
+      a口口口口口口口口口口口口口口口口                           |
+      aa口口口口口口口口口口口口口口口口                          |
+      aaa口口口口口口口口口口口口口口口口                         |
+      aaaa口口口口口口口口口口口口口口口口                        |
+                                                                  |
+    ]])
+
+    command('20vnew')
+    screen:expect([[
+      ^                    │口口口口口口口口口口口口口口口口       |
+      {1:~                   }│a口口口口口口口口口口口口口口口口      |
+      {1:~                   }│aa口口口口口口口口口口口口口口口口     |
+      {1:~                   }│aaa口口口口口口口口口口口口口口口口    |
+      {6:[No Name]            }{7:[No Name] [+]                          }|
+                                                                  |
+    ]])
+  end)
+
   it('0xffff is shown as 4 hex digits', function()
     command([[call setline(1, "\uFFFF!!!")]])
     feed('$')