Syntax: Fix conditions for syntax test handling

Fixes #365
Closes #366

plugins/lib/__init__.py

@@ -30,3 +30,15 @@ def wrapper(*args, **kwargs):
         return (ret, sublime.INHIBIT_WORD_COMPLETIONS) if ret is not None else None
 
     return wrapper
+
+
+def path_is_relative_to(path, *other):
+    """Check whether a `pathlib.Path` is relative to another Path-like.
+
+    Backport of Python 3.9's `pathlib.PurePath.is_relative_to`.
+    """
+    try:
+        path.relative_to(*other)
+        return True
+    except ValueError:
+        return False

plugins/syntaxtest_dev.py

@@ -2,13 +2,14 @@
 import logging
 import re
 from os import path
+from pathlib import Path
 
 import sublime
 import sublime_plugin
 
 from sublime_lib.flags import RegionOption
 
-from .lib import get_setting
+from .lib import get_setting, path_is_relative_to
 
 __all__ = (
     'SyntaxTestHighlighterListener',
@@ -435,7 +436,17 @@ class AssignSyntaxTestSyntaxListener(sublime_plugin.EventListener):
     PLAIN_TEXT = "Packages/Text/Plain text.tmLanguage"
 
     def on_load(self, view):
-        if view.size() == 0 and view.file_name().startswith(sublime.packages_path() + '/'):
+        file_name = view.file_name()
+        if not file_name:
+            return
+        file_path = Path(file_name)
+        if (
+            not file_path.name.startswith("syntax_test_")
+            or not path_is_relative_to(file_path, sublime.packages_path())
+        ):
+            return
+
+        if view.size() == 0:
             logger.debug("Delaying on_load because view was empty")
             sublime.set_timeout(lambda: self._on_load(view), 100)
         else: