[Linux] resort to "-dark" suffix for detecting platform brightness

Some users have reported that toggling between light and dark system
theme results in Flutter detecting the opposite theme. This is because
the window text color, which is used to calculate the brightness, may
not have been yet refreshed when the theme change signal is emitted.

This commit replaces the unreliable (and untestable) window text color
-based theme detection with the simpler alternative proposed in
flutter#25535 to check for a "-dark" suffix in the theme name,
which is used by convention in dark GTK theme names (Yaru vs. Yaru-dark,
Adwaita vs. Adwaita-dark, Pop vs. Pop-dark, ...).

Ref: flutter/flutter#101438

Author: jpnurmi

shell/platform/linux/fl_settings_plugin.cc

@@ -5,8 +5,6 @@
 #include "flutter/shell/platform/linux/fl_settings_plugin.h"
 
 #include <gmodule.h>
-#include <gtk/gtk.h>
-#include <math.h>
 
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_basic_message_channel.h"
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_json_message_codec.h"
@@ -21,9 +19,9 @@ static constexpr char kPlatformBrightnessDark[] = "dark";
 static constexpr char kDesktopInterfaceSchema[] = "org.gnome.desktop.interface";
 static constexpr char kDesktopTextScalingFactorKey[] = "text-scaling-factor";
 static constexpr char kDesktopClockFormatKey[] = "clock-format";
+static constexpr char kDesktopGtkThemeKey[] = "gtk-theme";
 static constexpr char kClockFormat24Hour[] = "24h";
-
-enum class Brightness { Light, Dark };
+static constexpr char kGtkThemeDarkSuffix[] = "-dark";
 
 struct _FlSettingsPlugin {
   GObject parent_instance;
@@ -37,54 +35,6 @@ struct _FlSettingsPlugin {
 
 G_DEFINE_TYPE(FlSettingsPlugin, fl_settings_plugin, G_TYPE_OBJECT)
 
-// The color brightness calculation has been adapted from theme_data.dart:
-// https://github.com/flutter/flutter/blob/8fe4cc79648a952f9c7e49a5248756c2ff98fa3b/packages/flutter/lib/src/material/theme_data.dart#L1470-L1488
-
-// See <https://www.w3.org/TR/WCAG20/#relativeluminancedef>.
-static gdouble linearize_color_component(gdouble component) {
-  if (component <= 0.03928) {
-    return component / 12.92;
-  }
-  return pow((component + 0.055) / 1.055, 2.4);
-}
-
-// See <https://en.wikipedia.org/wiki/Relative_luminance>.
-gdouble compute_luminance(GdkRGBA* color) {
-  gdouble r = linearize_color_component(color->red);
-  gdouble g = linearize_color_component(color->green);
-  gdouble b = linearize_color_component(color->blue);
-  return 0.2126 * r + 0.7152 * g + 0.0722 * b;
-}
-
-static Brightness estimate_brightness_for_color(GdkRGBA* color) {
-  gdouble relative_luminance = compute_luminance(color);
-
-  // See <https://www.w3.org/TR/WCAG20/#contrast-ratiodef> and
-  // <https://material.io/go/design-theming#color-color-palette>.
-  const gdouble kThreshold = 0.15;
-  if ((relative_luminance + 0.05) * (relative_luminance + 0.05) > kThreshold) {
-    return Brightness::Light;
-  }
-  return Brightness::Dark;
-}
-
-static bool is_dark_theme() {
-  // GTK doesn't have a specific flag for dark themes, so we check if the
-  // style text color is light or dark
-  GList* windows = gtk_window_list_toplevels();
-  if (windows == nullptr) {
-    return false;
-  }
-
-  GtkWidget* window = GTK_WIDGET(windows->data);
-  g_list_free(windows);
-
-  GdkRGBA text_color;
-  GtkStyleContext* style = gtk_widget_get_style_context(window);
-  gtk_style_context_get_color(style, GTK_STATE_FLAG_NORMAL, &text_color);
-  return estimate_brightness_for_color(&text_color) == Brightness::Light;
-}
-
 // Sends the current settings to the Flutter engine.
 static void update_settings(FlSettingsPlugin* self) {
   gdouble scaling_factor = 1.0;
@@ -97,10 +47,12 @@ static void update_settings(FlSettingsPlugin* self) {
     g_autofree gchar* clock_format =
         g_settings_get_string(self->interface_settings, kDesktopClockFormatKey);
     always_use_24hr = g_strcmp0(clock_format, kClockFormat24Hour) == 0;
-  }
 
-  if (is_dark_theme()) {
-    platform_brightness = kPlatformBrightnessDark;
+    g_autofree gchar* gtk_theme =
+        g_settings_get_string(self->interface_settings, kDesktopGtkThemeKey);
+    if (g_str_has_suffix(gtk_theme, kGtkThemeDarkSuffix)) {
+      platform_brightness = kPlatformBrightnessDark;
+    }
   }
 
   g_autoptr(FlValue) message = fl_value_new_map();