[lvgl] Honor esphome build_flags when generating lv_conf.h (#19743)

Co-authored-by: Clyde Stubbs <2366188+clydebarrow@users.noreply.github.com>

esphome/components/lvgl/__init__.py

@@ -31,6 +31,7 @@ from esphome.components.psram import DOMAIN as PSRAM_DOMAIN
 import esphome.config_validation as cv
 from esphome.const import (
     CONF_BUFFER_SIZE,
+    CONF_BUILD_FLAGS,
     CONF_ESPHOME,
     CONF_GROUP,
     CONF_ID,
@@ -169,11 +170,17 @@ def generate_lv_conf_h():
     all_defines = set(
         df.LV_DEFINES + tuple(f"LV_USE_{w.upper()}" for w in WIDGET_TYPES)
     )
-    build_flags = (
-        CORE.config[CONF_ESPHOME].get(CONF_PLATFORMIO_OPTIONS).get("build_flags", [])
+    esphome_config = CORE.config[CONF_ESPHOME]
+    # User build flags come from esphome->build_flags and from the deprecated
+    # esphome->platformio_options->build_flags (a string or a list).
+    # Remove before 2026.12.0
+
+    pio_build_flags = esphome_config.get(CONF_PLATFORMIO_OPTIONS, {}).get(
+        CONF_BUILD_FLAGS, []
     )
-    if not isinstance(build_flags, list):
-        build_flags = [build_flags]
+    if not isinstance(pio_build_flags, list):
+        pio_build_flags = [pio_build_flags]
+    build_flags = [*esphome_config.get(CONF_BUILD_FLAGS, []), *pio_build_flags]
     # Extract define names from build flags like '-DLV_USE_CHART=1', '-D LV_USE_CHART',
     # or multiple defines in one string.
     define_pattern = r'-D\s*([A-Z_][A-Z0-9_]*)(?:=[^\s\'"\]]*)?'

tests/component_tests/lvgl/test_lv_conf_build_flags.py

@@ -0,0 +1,59 @@
+"""``generate_lv_conf_h()`` writes ``#define LV_... 0`` for every LVGL option
+the configuration does not use, so ``lv_conf.h`` must leave alone the options
+the user defines in build flags. Otherwise the header, which is included after
+the compiler ``-D`` flags, turns the option off again.
+
+Build flags can come from ``esphome: build_flags:`` or from the deprecated
+``esphome: platformio_options: build_flags:``; both must be honoured.
+"""
+
+from __future__ import annotations
+
+import logging
+
+import pytest
+
+from esphome.components.lvgl import defines as df, generate_lv_conf_h
+from esphome.const import CONF_BUILD_FLAGS, CONF_ESPHOME, CONF_PLATFORMIO_OPTIONS
+from esphome.core import CORE
+
+
+def _set_esphome_config(
+    build_flags: list[str] | None = None,
+    pio_build_flags: list[str] | str | None = None,
+) -> None:
+    pio_options = {} if pio_build_flags is None else {"build_flags": pio_build_flags}
+    CORE.config = {
+        CONF_ESPHOME: {
+            CONF_PLATFORMIO_OPTIONS: pio_options,
+            CONF_BUILD_FLAGS: build_flags or [],
+        }
+    }
+
+
+def test_unused_define_is_disabled_without_build_flag() -> None:
+    _set_esphome_config()
+    assert "#define LV_USE_OBSERVER 0" in generate_lv_conf_h().splitlines()
+
+
+@pytest.mark.parametrize(
+    "flags",
+    [
+        {"build_flags": ["-DLV_USE_OBSERVER=1"]},
+        {"build_flags": ["-D LV_USE_OBSERVER"]},
+        {"pio_build_flags": ["-DLV_USE_OBSERVER=1"]},
+        {"pio_build_flags": "-DLV_USE_OBSERVER=1"},
+    ],
+    ids=["esphome", "esphome-spaced", "platformio_options", "platformio_options-str"],
+)
+def test_build_flag_define_is_not_disabled(flags: dict) -> None:
+    _set_esphome_config(**flags)
+    assert "#define LV_USE_OBSERVER 0" not in generate_lv_conf_h().splitlines()
+
+
+def test_esphome_build_flag_clash_warns(caplog: pytest.LogCaptureFixture) -> None:
+    _set_esphome_config(build_flags=["-DLV_USE_ARC=1"])
+    df.add_define("LV_USE_ARC")
+    with caplog.at_level(logging.WARNING):
+        generate_lv_conf_h()
+    assert "LV_USE_ARC" in caplog.text