[lvgl] Use contextvars to improve lvgl scheduling (#19242)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

tests/component_tests/lvgl/test_multi_instance_theme.py

@@ -0,0 +1,44 @@
+"""Regression test: a widget's theme style must be attached regardless of
+which LVGL instance declares theme: and which instance's widgets are built
+first.
+
+theme_to_code() runs once per LVGL instance, interleaved with that instance's
+own add_widgets(). A widget built by an earlier instance, before any instance
+has declared theming for its type, must still pick up a later instance's
+theme: declaration for that type - including on the later instance's own
+widgets, which get_widget_theme_styles()'s memoisation can poison too.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from esphome.__main__ import generate_cpp_contents
+from esphome.config import read_config
+from esphome.core import CORE
+
+
+@pytest.fixture(scope="module")
+def main_cpp(request: pytest.FixtureRequest) -> str:
+    config_path = (
+        Path(request.fspath).parent / "config" / "multi_instance_theme_test.yaml"
+    )
+    original_path = CORE.config_path
+    try:
+        CORE.config_path = config_path
+        CORE.config = read_config({})
+        generate_cpp_contents(CORE.config)
+        return CORE.cpp_main_section
+    finally:
+        CORE.config_path = original_path
+        CORE.reset()
+
+
+def test_earlier_instance_widget_is_themed(main_cpp: str) -> None:
+    assert "lv_obj_add_style(label_0, _lv_theme_style_label_main_default," in main_cpp
+
+
+def test_later_instance_widget_is_themed(main_cpp: str) -> None:
+    assert "lv_obj_add_style(label_1, _lv_theme_style_label_main_default," in main_cpp