[Theme] Pass application palette when generating disabled icon pixmap (#7831)

Co-authored-by: Lukas Brübach <44814898+BruebachL@users.noreply.github.com>
Co-authored-by: sentry[bot] <39604003+sentry[bot]@users.noreply.github.com>

tests/theme_migration/theme_icon_render_test.cpp

@@ -619,6 +619,34 @@ TEST_F(ThemeIconRenderTest, BadgeOnAuthoredArtUsesTheAssetPath)
     EXPECT_EQ(countOpaquePixels(pixmap, tone.darker(300)), 0);
 }
 
+/**
+ * @brief Disabled authored art is generated by the style without crashing.
+ *
+ * Art that carries its own colors is dimmed through
+ * QStyle::generatedIconPixmap, which dereferences the style option's palette;
+ * passing no option crashed in QPalette::brush.
+ */
+TEST_F(ThemeIconRenderTest, DisabledAuthoredArtIsGeneratedByTheStyle)
+{
+    QTemporaryDir themeDir;
+    ASSERT_TRUE(themeDir.isValid());
+    QFile file(QDir(themeDir.path()).absoluteFilePath(QStringLiteral("plain.svg")));
+    ASSERT_TRUE(file.open(QIODevice::WriteOnly));
+    file.write(svgWith("#336699"));
+    file.close();
+    QDir::addSearchPath(QStringLiteral("theme"), themeDir.path());
+
+    const QIcon icon = loadThemedIcon(QStringLiteral("plain"));
+    const QPixmap normal = icon.pixmap(QSize(32, 32));
+    const QPixmap disabled = icon.pixmap(QSize(32, 32), QIcon::Disabled);
+
+    ASSERT_FALSE(normal.isNull());
+    ASSERT_FALSE(disabled.isNull());
+    EXPECT_GT(countOpaquePixels(normal, QColor(QStringLiteral("#336699"))), 0);
+    // The style dims the art, so the authored color no longer appears verbatim.
+    EXPECT_EQ(countOpaquePixels(disabled, QColor(QStringLiteral("#336699"))), 0);
+}
+
 /** @brief Disabled keeps the shape language but drops back to the line glyph. */
 TEST_F(ThemeIconRenderTest, DisabledBadgeFallsBackToLineGlyph)
 {