fix(extmarks): handle overwriting right half of wide char (#26951)

src/nvim/drawline.c

@@ -206,6 +206,9 @@ static void margin_columns_win(win_T *wp, int *left_col, int *right_col)
 /// Handles composing chars
 static int line_putchar(buf_T *buf, const char **pp, schar_T *dest, int maxcells, int vcol)
 {
+  // Caller should handle overwriting the left half of a double-width char.
+  assert(dest[0] != 0);
+
   const char *p = *pp;
   int cells = utf_ptr2cells(p);
   int c_len = utfc_ptr2len(p);
@@ -219,6 +222,7 @@ static int line_putchar(buf_T *buf, const char **pp, schar_T *dest, int maxcells
     cells = MIN(tabstop_padding(vcol, buf->b_p_ts, buf->b_p_vts_array), maxcells);
   }
 
+  // When overwriting the left half of a double-width char, clear the right half.
   if (cells < maxcells && dest[cells] == 0) {
     dest[cells] = schar_from_ascii(' ');
   }
@@ -330,8 +334,15 @@ static int draw_virt_text_item(buf_T *buf, int col, VirtText vt, HlMode hl_mode,
     } else {
       attr = virt_attr;
     }
-    schar_T dummy[2];
+    schar_T dummy[2] = { schar_from_ascii(' '), schar_from_ascii(' ') };
     int maxcells = max_col - col;
+    // When overwriting the right half of a double-width char, clear the left half.
+    if (linebuf_char[col] == 0) {
+      assert(col > 0);
+      linebuf_char[col - 1] = schar_from_ascii(' ');
+      // Clear the right half as well for the assertion in line_putchar().
+      linebuf_char[col] = schar_from_ascii(' ');
+    }
     int cells = line_putchar(buf, &p, through ? dummy : &linebuf_char[col],
                              maxcells, vcol);
     for (int c = 0; c < cells; c++) {
@@ -867,7 +878,11 @@ static void win_line_start(win_T *wp, winlinevars_T *wlv, bool save_extra)
   wlv->col = 0;
   wlv->off = 0;
   wlv->need_lbr = false;
-  memset(linebuf_vcol, -1, (size_t)wp->w_grid.cols * sizeof(*linebuf_vcol));
+  for (int i = 0; i < wp->w_grid.cols; i++) {
+    linebuf_char[i] = schar_from_ascii(' ');
+    linebuf_attr[i] = -1;
+    linebuf_vcol[i] = -1;
+  }
 }
 
 static void fix_for_boguscols(winlinevars_T *wlv)

test/functional/ui/decorations_spec.lua

@@ -2082,7 +2082,37 @@ describe('extmark decorations', function()
     ]]}
   end)
 
-  it('works with double width char and rightleft', function()
+  it('virtual text overwrites double-width char properly', function()
+    screen:try_resize(50, 3)
+    insert('abcdefghij口klmnopqrstu口vwx口yz')
+    feed('0')
+    meths.buf_set_extmark(0, ns, 0, 0, { virt_text = {{'!!!!!', 'Underlined'}}, virt_text_win_col = 11 })
+    screen:expect{grid=[[
+      ^abcdefghij {28:!!!!!}opqrstu口vwx口yz                  |
+      {1:~                                                 }|
+                                                        |
+    ]]}
+    feed('8x')
+    screen:expect{grid=[[
+      ^ij口klmnopq{28:!!!!!} vwx口yz                          |
+      {1:~                                                 }|
+                                                        |
+    ]]}
+    feed('3l5x')
+    screen:expect{grid=[[
+      ij口^pqrstu {28:!!!!!} yz                               |
+      {1:~                                                 }|
+                                                        |
+    ]]}
+    feed('5x')
+    screen:expect{grid=[[
+      ij口^u口vwx {28:!!!!!}                                  |
+      {1:~                                                 }|
+                                                        |
+    ]]}
+  end)
+
+  it('virtual text works with double-width char and rightleft', function()
     screen:try_resize(50, 3)
     insert('abcdefghij口klmnopqrstu口vwx口yz')
     feed('0')