fix: address CodeRabbit findings on the style-editor/font-selector PR

- Fix _load_font_sized double-wrapping the (font, size) tuple on the
  missing-font path, which handed callers a tuple instead of a font.
- Fix _set_nested_value skipping an explicit None when the key already
  existed, which silently kept stale overrides when a user cleared a
  nullable per-mode field or blanked all channels of an indexed color.
- Preserve BDF scalable/native_size metadata through fetchFontCatalog's
  catalog-format mapping so maxFixedSize filtering actually applies.
- Stop caching an empty array on a failed font-catalog fetch so a later
  call can retry instead of being stuck with the failed result.
- Keep a saved font selected in the style editor even when it no longer
  fits a newly declared maxFixedSize, instead of silently deselecting it.
- Don't drop in-progress user edits to fallback fields when a plugin
  widget finishes loading asynchronously and takes over the form.
- Tighten the removed font-override endpoint test to assert 405, not
  just != 200.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/test_element_style.py

@@ -359,6 +359,19 @@ class TestDegradation:
         # The override IS honored as forced, but the face degrades safely.
         assert style.user_forced
         assert style.font is not None
+        assert not isinstance(style.font, tuple)
+
+    def test_missing_font_load_returns_a_font_not_a_nested_tuple(self):
+        # Regression: the missing-font path in _load_font_sized used to wrap
+        # _load_fallback_font's own (font, size) tuple a second time, so
+        # load_font() returned a (font, size) tuple where callers (draw.text,
+        # layout math) expect a font object.
+        font, size = _load_font_sized("definitely-not-a-real-font.ttf", 8)
+        assert not isinstance(font, tuple)
+        assert size == 8
+
+        single = load_font("definitely-not-a-real-font.ttf", 8)
+        assert not isinstance(single, tuple)
 
     def test_empty_defaults_treats_config_as_reference_to_classic(self):
         # No schema defaults at all: a config value equal to the classic

test/test_web_api.py

@@ -825,10 +825,10 @@ class TestFontsAPI:
         # 405, not 404: the path still matches DELETE /fonts/<font_family>,
         # which now reads "overrides" as a font name. Nothing is routed to a
         # handler for GET or POST, which is what matters here.
-        assert client.get('/api/v3/fonts/overrides').status_code != 200
+        assert client.get('/api/v3/fonts/overrides').status_code == 405
         assert client.post('/api/v3/fonts/overrides',
                            data=json.dumps({}),
-                           content_type='application/json').status_code != 200
+                           content_type='application/json').status_code == 405
 
 
 class TestAPIErrorHandling:

test/web_interface/test_api_v3_helpers.py

@@ -25,6 +25,7 @@ from web_interface.blueprints.api_v3 import (  # noqa: E402
     _parse_form_value,
     _get_schema_property,
     _set_nested_value,
+    _SKIP_FIELD,
 )
 
 
@@ -221,12 +222,21 @@ class TestSetNestedValue:
         _set_nested_value(config, "fifa.world.enabled", True)
         assert config == {"fifa.world": {"enabled": True}}
 
-    def test_none_does_not_overwrite_existing(self):
+    def test_none_overwrites_existing(self):
+        # Regression: None is a real value here (the per-mode "inherit the
+        # base" override for a nullable field, or a blank indexed color
+        # channel) and must replace whatever was already stored. Only the
+        # _SKIP_FIELD sentinel means "leave it alone" -- see the next test.
         config = {"a": 1}
         _set_nested_value(config, "a", None)
-        assert config == {"a": 1}
+        assert config == {"a": None}
 
     def test_none_sets_missing_key(self):
         config = {}
         _set_nested_value(config, "a", None)
         assert config == {"a": None}
+
+    def test_skip_field_does_not_overwrite_existing(self):
+        config = {"a": 1}
+        _set_nested_value(config, "a", _SKIP_FIELD)
+        assert config == {"a": 1}