🐛 FIX: count lines, not characters, for {include} :start-after:

myst_parser/mocking.py

@@ -442,7 +442,7 @@ def run(self) -> list[nodes.Element]:
                     f'Directive "{self.name}"; option "{split_on_type}": text not found "{split_on}".',
                 )
             if split_on_type == "start-after":
-                startline += split_index + len(split_on)
+                startline += file_content[: split_index + len(split_on)].count("\n")
                 file_content = file_content[split_index + len(split_on) :]
             else:
                 file_content = file_content[:split_index]

tests/test_renderers/fixtures/mock_include_errors.md

@@ -21,3 +21,22 @@ Error in include file:
 .
 tmpdir/bad.md:2: (WARNING/2) Unknown interpreted text role "a". [myst.role_unknown]
 .
+
+Error in include file with start-after:
+.
+```{include} bad_after.md
+:start-after: START-HERE
+```
+.
+tmpdir/bad_after.md:5: (WARNING/2) Unknown interpreted text role "a". [myst.role_unknown]
+.
+
+Error in include file with start-line and start-after:
+.
+```{include} bad_after.md
+:start-line: 1
+:start-after: START-HERE
+```
+.
+tmpdir/bad_after.md:5: (WARNING/2) Unknown interpreted text role "a". [myst.role_unknown]
+.

tests/test_renderers/test_include_directive.py

@@ -41,6 +41,9 @@ def test_errors(file_params, tmp_path, monkeypatch):
     monkeypatch.chdir(tmp_path)
 
     tmp_path.joinpath("bad.md").write_text("{a}`b`")
+    tmp_path.joinpath("bad_after.md").write_text(
+        "line one\nline two\nSTART-HERE\nline four\n{a}`b`"
+    )
 
     report_stream = StringIO()
     publish_doctree(