Fix in Flex layout interacting with float elements.

src/eepp/ui/flexlayouter.cpp

@@ -1222,6 +1222,14 @@ void FlexLayouter::updateLayout() {
 	if ( widget->isInline() || mPacking )
 		return;
 
+	RichText* richText = widget->isType( UI_TYPE_RICHTEXT )
+									   ? widget->asType<UIRichText>()->getRichTextPtr()
+									   : nullptr;
+	bool preserveFloatConstrainedBFCWidth =
+		widget->establishesBlockFormattingContext() &&
+		widget->getLayoutWidthPolicy() == SizePolicy::MatchParent && richText != nullptr &&
+		!richText->getExternalFloatExclusions().empty();
+
 	mPacking = true;
 
 	mContainer->beginAttributesTransaction();
@@ -1230,7 +1238,7 @@ void FlexLayouter::updateLayout() {
 		auto* parent = mContainer->getParent();
 		bool parentIsFlexContainer = parent && parent->isType( UI_TYPE_HTML_WIDGET ) &&
 									 static_cast<UIHTMLWidget*>( parent )->isFlex();
-		if ( !parentIsFlexContainer )
+		if ( !parentIsFlexContainer && !preserveFloatConstrainedBFCWidth )
 			setMatchParentIfNeededVerticalGrowth();
 	}
 
@@ -1333,6 +1341,8 @@ void FlexLayouter::updateLayout() {
 														containerPadding.Right );
 				else if ( widthPolicy == SizePolicy::Fixed )
 					mContainer->setInternalPixelsWidth( containerWidth );
+				else if ( preserveFloatConstrainedBFCWidth )
+					mContainer->setInternalPixelsWidth( containerWidth );
 				else
 					mContainer->setInternalPixelsWidth(
 						mContainer->getParent()
@@ -1355,6 +1365,8 @@ void FlexLayouter::updateLayout() {
 				mContainer->setInternalPixelsWidth( totW );
 			else if ( widthPolicy == SizePolicy::Fixed )
 				mContainer->setInternalPixelsWidth( containerWidth );
+			else if ( preserveFloatConstrainedBFCWidth )
+				mContainer->setInternalPixelsWidth( containerWidth );
 			else
 				mContainer->setInternalPixelsWidth(
 					mContainer->getParent() ? mContainer->getParent()->getPixelsSize().getWidth()
@@ -1403,6 +1415,13 @@ void FlexLayouter::updateLayout() {
 
 	applyLayout( lines, mainAxis, crossAxis, containerPadding, containerWidth, containerHeight,
 				 widthPolicy, heightPolicy );
+	if ( preserveFloatConstrainedBFCWidth ) {
+		// The parent RichText stream has already resolved this match-parent BFC against active
+		// floats and placed it into a narrowed atomic fragment. Flex layout still needs to run
+		// using that used width, but its normal match-parent finalization would expand back to
+		// the full parent content box and overlap the float.
+		mContainer->setInternalPixelsWidth( containerWidth );
+	}
 
 	// CSS Flexbox §8.5: store the container's baseline for use by outer flex containers.
 	// For row/row-reverse, baseline = first flex line's baseline offset from content top.

src/tests/unit_tests/uihtml_tests.cpp

@@ -3745,6 +3745,70 @@ UTEST( UIHTML, PaddedHeaderWithNestedFloatsDoesNotOverlapClearedMain ) {
 	Engine::destroySingleton();
 }
 
+UTEST( UIHTML, FlexInfobarAvoidsRightFloat ) {
+	auto win = Engine::instance()->createWindow(
+		WindowSettings( 800, 600, "flex infobar avoids right float", WindowStyle::Default,
+						WindowBackend::Default, 32, {}, 1, false, true ),
+		ContextSettings( false, 0, 0, GLv_default, true, false ) );
+	FileSystem::changeWorkingDirectory( Sys::getProcessPath() );
+
+	UI::UISceneNode* sceneNode = init_test_inline_block();
+	sceneNode->loadLayoutFromString( HTMLFormatter::HTMLtoXML( R"html(
+		<html>
+			<head>
+				<style>
+					body {
+						margin: 0;
+						font: normal 12px verdana;
+					}
+					.side {
+						float: right;
+						width: 300px;
+						height: 120px;
+						margin: 0 5px;
+					}
+					.content {
+						margin: 7px 5px 0 5px;
+					}
+					.infobar {
+						display: flex;
+						overflow: hidden;
+						box-sizing: border-box;
+						height: 124px;
+						margin: 5px;
+						padding: 0;
+					}
+				</style>
+			</head>
+			<body>
+				<div class="side" id="side"></div>
+				<div class="content" id="content">
+					<section class="infobar" id="infobar">
+						<a>Welcome to Reddit.</a>
+					</section>
+				</div>
+			</body>
+		</html>
+	)html" ) );
+	win->getInput()->update();
+	SceneManager::instance()->update();
+	sceneNode->updateDirtyLayouts();
+	sceneNode->updateDirtyLayouts();
+
+	auto* side = sceneNode->getRoot()->find( "side" )->asType<UIHTMLWidget>();
+	auto* infobar = sceneNode->getRoot()->find( "infobar" )->asType<UIHTMLWidget>();
+	ASSERT_TRUE( side != nullptr );
+	ASSERT_TRUE( infobar != nullptr );
+
+	const Float sideLeft = side->convertToWorldSpace( { 0, 0 } ).x;
+	const Float infobarRight =
+		infobar->convertToWorldSpace( { 0, 0 } ).x + infobar->getPixelsSize().getWidth();
+
+	EXPECT_LE( infobarRight, sideLeft + 0.5f );
+
+	Engine::destroySingleton();
+}
+
 UTEST( UIHTML, AnchorsSizing ) {
 	auto win = Engine::instance()->createWindow(
 		WindowSettings( 1024, 653, "anchors sizing", WindowStyle::Default, WindowBackend::Default,