Use a ResizeObserver to determine default font-size for text tracks

third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp

Index: third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
diff --git a/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp b/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
index 1ffbea3e0c9c1ed65bd7a6a1e97c1ce6aadcdea2..dcfed8d0d7a7bdc7bad46082c06163c3c16efc7c 100644
--- a/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
+++ b/third_party/WebKit/Source/core/html/track/TextTrackContainer.cpp
@@ -29,24 +29,97 @@
 
 #include "core/html/track/TextTrackContainer.h"
 
+#include "core/dom/ResizeObserver.h"
+#include "core/dom/ResizeObserverCallback.h"
+#include "core/dom/ResizeObserverEntry.h"
 #include "core/html/HTMLVideoElement.h"
 #include "core/html/track/CueTimeline.h"
-#include "core/layout/LayoutTextTrackContainer.h"
+#include "core/layout/LayoutBlockFlow.h"
+#include "core/layout/LayoutVideo.h"
 
 namespace blink {
 
+namespace {
+
+class VideoElementResizeCallback final : public ResizeObserverCallback {
+ public:
+  VideoElementResizeCallback(TextTrackContainer& container)
+      : ResizeObserverCallback(), text_track_container_(container) {}
+
+  void handleEvent(const HeapVector<Member<ResizeObserverEntry>>& entries,
+                   ResizeObserver*) override {
+    DCHECK_EQ(entries.size(), 1u);
+    DCHECK(isHTMLVideoElement(entries[0]->target()));
+    text_track_container_->UpdateDefaultFontSize(
+        entries[0]->target()->GetLayoutObject());
+  }
+
+  DEFINE_INLINE_VIRTUAL_TRACE() {
+    visitor->Trace(text_track_container_);
+    ResizeObserverCallback::Trace(visitor);
+  }
+
+ private:
+  Member<TextTrackContainer> text_track_container_;
+};
+
+}  // namespace
+
 TextTrackContainer::TextTrackContainer(Document& document)
-    : HTMLDivElement(document) {}
+    : HTMLDivElement(document), default_font_size_(0) {}
 
-TextTrackContainer* TextTrackContainer::Create(Document& document) {
-  TextTrackContainer* element = new TextTrackContainer(document);
+DEFINE_TRACE(TextTrackContainer) {
+  visitor->Trace(video_size_observer_);
+  HTMLDivElement::Trace(visitor);
+}
+
+TextTrackContainer* TextTrackContainer::Create(
+    HTMLMediaElement& media_element) {
+  TextTrackContainer* element =
+      new TextTrackContainer(media_element.GetDocument());
   element->SetShadowPseudoId(
       AtomicString("-webkit-media-text-track-container"));
+  if (isHTMLVideoElement(media_element))
+    element->ObserveSizeChanges(media_element);
   return element;
 }
 
 LayoutObject* TextTrackContainer::CreateLayoutObject(const ComputedStyle&) {
-  return new LayoutTextTrackContainer(this);
+  return new LayoutBlockFlow(this);
+}
+
+void TextTrackContainer::ObserveSizeChanges(Element& element) {
+  video_size_observer_ = ResizeObserver::Create(
+      GetDocument(), new VideoElementResizeCallback(*this));
+  video_size_observer_->observe(&element);
+}
+
+void TextTrackContainer::UpdateDefaultFontSize(
+    LayoutObject* media_layout_object) {
+  if (!media_layout_object || !media_layout_object->IsVideo())
+    return;
+  // FIXME: The video size is used to calculate the font size (a workaround
+  // for lack of per-spec vh/vw support) but the whole media element is used
+  // for cue rendering. This is inconsistent. See also the somewhat related
+  // spec bug: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28105
+  LayoutSize video_size =
+      ToLayoutVideo(*media_layout_object).ReplacedContentRect().size();
+  LayoutUnit smallest_dimension =
+      std::min(video_size.Height(), video_size.Width());
+  float font_size = smallest_dimension * 0.05f;
+
+  // Avoid excessive FP precision issue.
+  // C11 5.2.4.2.2:9 requires assignment and cast to remove extra precision, but
+  // the behavior is currently not portable. fontSize may have precision higher
+  // than m_fontSize thus straight comparison can fail despite they cast to the
+  // same float value.
+  volatile float& current_font_size = default_font_size_;
+  float old_font_size = current_font_size;
+  current_font_size = font_size;
+  if (current_font_size == old_font_size)
+    return;
+  SetInlineStyleProperty(CSSPropertyFontSize, default_font_size_,
+                         CSSPrimitiveValue::UnitType::kPixels);
 }
 
 void TextTrackContainer::UpdateDisplay(HTMLMediaElement& media_element,