fix(highlight): ensure extmark range is within buffer bounds #35928

Adds an additional check for the case when end_col = 0, addressing
https://github.com/neovim/neovim/issues/35814#issuecomment-3340709532.

Validation is now localized to the highlighter without affecting the C API.

runtime/lua/vim/treesitter/highlighter.lua

@@ -443,8 +443,12 @@ local function on_range_impl(
           local url = get_url(match, buf, capture, metadata)
 
           if hl and not on_conceal and (not on_spell or spell ~= nil) then
+            -- Workaround for #35814: ensure the range is within buffer bounds,
+            -- allowing the last line if end_col is 0.
+            -- TODO(skewb1k): investigate a proper concurrency-safe handling of extmarks.
+            if (end_row + (end_col > 0 and 1 or 0)) <= api.nvim_buf_line_count(buf) then
               api.nvim_buf_set_extmark(buf, ns, start_row, start_col, {
-              end_line = end_row,
+                end_row = end_row,
                 end_col = end_col,
                 hl_group = hl,
                 ephemeral = true,
@@ -455,6 +459,7 @@ local function on_range_impl(
                 _subpriority = subtree_counter,
               })
             end
+          end
 
           if
             (metadata.conceal_lines or metadata[capture] and metadata[capture].conceal_lines)

src/nvim/api/extmark.c

@@ -813,8 +813,7 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id, Integer line, Integer
       len = opts->ephemeral ? MAXCOL : ml_get_buf_len(buf, (linenr_T)line2 + 1);
     } else if (line2 == buf->b_ml.ml_line_count) {
       // We are trying to add an extmark past final newline
-      // TODO(skewb1k): workaround for #35814. Investigate proper concurrency-safe handling of extmarks.
-      len = opts->ephemeral ? MAXCOL : 0;
+      len = 0;
     } else {
       // reuse len from before
       line2 = (int)line;