Accounted for a couple edge cases.

Author: Thomas Smith

st3/sublime_lib/resource_path.py

@@ -108,10 +108,13 @@ def from_file_path(cls, file_path):
 
         for base in get_installed_resource_roots():
             try:
-                package, *rest = file_path.relative_to(base).parts
+                rel = file_path.relative_to(base).parts
             except ValueError:
                 pass
             else:
+                if rel == ():
+                    return cls('Packages')
+                package, *rest = rel
                 package_path = cls('Packages', package)
                 if package_path.suffix == '.sublime-package':
                     return package_path.with_suffix('').joinpath(*rest)

tests/test_resource_path.py

@@ -58,6 +58,12 @@ def test_from_file_path_installed_packages_not_installed(self):
                 Path(sublime.installed_packages_path(), 'test_package', 'foo.py')
             ),
 
+    def test_from_file_path_installed_packages_root(self):
+        self.assertEqual(
+            ResourcePath.from_file_path(Path(sublime.installed_packages_path())),
+            ResourcePath("Packages")
+        )
+
     def test_from_file_path_default_packages(self):
         self.assertEqual(
             ResourcePath.from_file_path(
@@ -68,6 +74,14 @@ def test_from_file_path_default_packages(self):
             ResourcePath("Packages/test_package/foo.py")
         )
 
+    def test_from_file_path_default_packages_root(self):
+        self.assertEqual(
+            ResourcePath.from_file_path(
+                Path(sublime.executable_path()).parent / 'Packages'
+            ),
+            ResourcePath("Packages")
+        )
+
     def test_from_file_path_error(self):
         with self.assertRaises(ValueError):
             ResourcePath.from_file_path(Path('/test_package')),