Compute shrink-to-fit width for flex blocks

Refs #552

Source/Core/Layout/ContainerBox.cpp

@@ -31,6 +31,7 @@
 #include "../../../Include/RmlUi/Core/Element.h"
 #include "../../../Include/RmlUi/Core/ElementScroll.h"
 #include "../../../Include/RmlUi/Core/Profiling.h"
+#include "FlexFormattingContext.h"
 #include "FormattingContext.h"
 #include "LayoutDetails.h"
 #include <algorithm>
@@ -274,7 +275,8 @@ float FlexContainer::GetShrinkToFitWidth() const
 	if (element->GetComputedValues().width().type == Style::Width::Type::Length)
 		return box.GetSize().x;
 
-	return 0.0f;
+	// Infer shrink-to-fit width from the intrinsic width of the element.
+	return FlexFormattingContext::GetMaxContentSize(element).x;
 }
 
 String FlexContainer::DebugDumpTree(int depth) const

Source/Core/Layout/FlexFormattingContext.cpp

@@ -118,6 +118,27 @@ UniquePtr<LayoutBox> FlexFormattingContext::Format(ContainerBox* parent_containe
 	return flex_container_box;
 }
 
+Vector2f FlexFormattingContext::GetMaxContentSize(Element *element) {
+	// A large but finite number is used here, because the flexbox formatting algorithm
+	// needs to round numbers, and it doesn't support infinities.
+	Vector2f infinity(10000.0f, 10000.0f);
+	RootBox root(infinity);
+	auto flex_container_box = MakeUnique<FlexContainer>(element, &root);
+
+	FlexFormattingContext context;
+	context.flex_container_box = flex_container_box.get();
+	context.element_flex = element;
+	context.flex_available_content_size = infinity;
+	context.flex_content_containing_block = infinity;
+	context.flex_max_size = infinity;
+
+	// Format the flexbox and all its children.
+	Vector2f flex_resulting_content_size, content_overflow_size;
+	float flex_baseline = 0.f;
+	context.Format(flex_resulting_content_size, content_overflow_size, flex_baseline);
+	return flex_resulting_content_size;
+}
+
 struct FlexItem {
 	// In the following, suffix '_a' means flex start edge while '_b' means flex end edge.
 	struct Size {

Source/Core/Layout/FlexFormattingContext.h

@@ -43,8 +43,12 @@ class FlexContainer;
 */
 class FlexFormattingContext final : public FormattingContext {
 public:
+	/// Formats a flex container element and its flex items according to flexbox layout rules.
 	static UniquePtr<LayoutBox> Format(ContainerBox* parent_container, Element* element, const Box* override_initial_box);
 
+	/// Computes max-content size for a flex container.
+	static Vector2f GetMaxContentSize(Element* element);
+
 private:
 	FlexFormattingContext() = default;
 

Source/Core/Layout/LayoutDetails.cpp

@@ -256,16 +256,16 @@ float LayoutDetails::GetShrinkToFitWidth(Element* element, Vector2f containing_b
 	LayoutDetails::BuildBox(box, containing_block, element, BuildBoxMode::UnalignedBlock);
 	LayoutDetails::GetDefiniteMinMaxHeight(min_height, max_height, element->GetComputedValues(), box, containing_block.y);
 
-	// Currently we don't support shrink-to-fit width for flexboxes or tables. Just return a zero-sized width.
+	// Currently we don't support shrink-to-fit width for inline flexboxes or tables. Just return a zero-sized width.
 	const Style::Display display = element->GetDisplay();
-	if (display == Style::Display::Flex || display == Style::Display::InlineFlex || display == Style::Display::Table ||
-		display == Style::Display::InlineTable)
+	if (display == Style::Display::InlineFlex || display == Style::Display::Table || display == Style::Display::InlineTable) {
 		return 0.f;
+	}
 
 	// Use a large size for the box content width, so that it is practically unconstrained. This makes the formatting
 	// procedure act as if under a maximum content constraint. Children with percentage sizing values may be scaled
 	// based on this width (such as 'width' or 'margin'), if so, the layout is considered undefined like in CSS 2.
-	const float max_content_constraint_width = containing_block.x + 1000.f;
+	const float max_content_constraint_width = containing_block.x + 10000.f;
 	box.SetContent({max_content_constraint_width, box.GetSize().y});
 
 	// First, format the element under the above generated box. Then we ask the resulting box for its shrink-to-fit
@@ -275,9 +275,13 @@ float LayoutDetails::GetShrinkToFitWidth(Element* element, Vector2f containing_b
 	RootBox root(Math::Max(containing_block, Vector2f(0.f)));
 	UniquePtr<LayoutBox> layout_box = FormattingContext::FormatIndependent(&root, element, &box, FormattingContextType::Block);
 
-	const float available_width = Math::Max(0.f, containing_block.x - box.GetSizeAcross(BoxDirection::Horizontal, BoxArea::Margin, BoxArea::Padding));
-
-	return Math::Min(available_width, layout_box->GetShrinkToFitWidth());
+	float shrink_to_fit_width = layout_box->GetShrinkToFitWidth();
+	if (containing_block.x >= 0) {
+		const float available_width = Math::Max(
+			0.f, containing_block.x - box.GetSizeAcross(BoxDirection::Horizontal, BoxArea::Margin, BoxArea::Padding));
+		shrink_to_fit_width = Math::Min(shrink_to_fit_width, available_width);
+	}
+	return shrink_to_fit_width;
 }
 
 ComputedAxisSize LayoutDetails::BuildComputedHorizontalSize(const ComputedValues& computed)