Media controls max width is the width of the viewport.

third_party/WebKit/Source/core/layout/LayoutMedia.cpp

 /*
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  *
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  *    notice, this list of conditions and the following disclaimer.
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  * THIS SOFTWARE IS PROVIDED BY APPLE COMPUTER, INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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 #include "core/layout/LayoutMedia.h"
 
+#include "core/frame/FrameHost.h"
+#include "core/frame/FrameView.h"
+#include "core/frame/VisualViewport.h"
 #include "core/html/HTMLMediaElement.h"
 #include "core/html/shadow/MediaControls.h"
 #include "core/layout/LayoutView.h"
+#include "core/page/Page.h"
 
 namespace blink {
 
 LayoutMedia::LayoutMedia(HTMLMediaElement* video) : LayoutImage(video) {
   setImageResource(LayoutImageResource::create());
 }
 
 LayoutMedia::~LayoutMedia() {}
 
 HTMLMediaElement* LayoutMedia::mediaElement() const {
   return toHTMLMediaElement(node());
 }
 
 void LayoutMedia::layout() {
   LayoutSize oldSize = contentBoxRect().size();
 
   LayoutImage::layout();
 
   LayoutRect newRect = contentBoxRect();
 
   LayoutState state(*this);
 
+  Optional<LayoutUnit> newPanelWidth;
+
 // Iterate the children in reverse order so that the media controls are laid
 // out before the text track container. This is to ensure that the text
 // track rendering has an up-to-date position of the media controls for
 // overlap checking, see LayoutVTTCue.
 #if ENABLE(ASSERT)
   bool seenTextTrackContainer = false;
 #endif
   for (LayoutObject* child = m_children.lastChild(); child;
        child = child->previousSibling()) {
 #if ENABLE(ASSERT)
     if (child->node()->isMediaControls())
       ASSERT(!seenTextTrackContainer);
     else if (child->node()->isTextTrackContainer())
       seenTextTrackContainer = true;
     else
       ASSERT_NOT_REACHED();
 #endif
 
+    // TODO(mlamouri): we miss some layouts because needsLayout returns false in
+    // some cases where we want to change the width of the controls because the
+    // visible viewport has changed for example.
     if (newRect.size() == oldSize && !child->needsLayout())
       continue;
 
+    LayoutUnit width = newRect.width();
+    if (child->node()->isMediaControls()) {
+      width = computePanelWidth(newRect);
+      if (width != oldSize.width())
+        newPanelWidth = width;
+    }
+
     LayoutBox* layoutBox = toLayoutBox(child);
     layoutBox->setLocation(newRect.location());
     // TODO(foolip): Remove the mutableStyleRef() and depend on CSS
     // width/height: inherit to match the media element size.
     layoutBox->mutableStyleRef().setHeight(Length(newRect.height(), Fixed));
-    layoutBox->mutableStyleRef().setWidth(Length(newRect.width(), Fixed));
+    layoutBox->mutableStyleRef().setWidth(Length(width, Fixed));
+
     layoutBox->forceLayout();
   }
 
   clearNeedsLayout();
 
   // Notify our MediaControls that a layout has happened.
   if (mediaElement() && mediaElement()->mediaControls() &&
-      newRect.width() != oldSize.width())
-    mediaElement()->mediaControls()->notifyPanelWidthChanged(newRect.width());
+      newPanelWidth.has_value()) {
+    mediaElement()->mediaControls()->notifyPanelWidthChanged(
+        newPanelWidth.value());
+  }
 }
 
 bool LayoutMedia::isChildAllowed(LayoutObject* child,
                                  const ComputedStyle&) const {
   // Two types of child layout objects are allowed: media controls
   // and the text track container. Filter children by node type.
   ASSERT(child->node());
 
   // The user agent stylesheet (mediaControls.css) has
   // ::-webkit-media-controls { display: flex; }. If author style
@@ skipping 33 matching lines @@
     element->notifyPositionMayHaveChanged(visibleRect);
 }
 
 void LayoutMedia::setRequestPositionUpdates(bool want) {
   if (want)
     view()->registerMediaForPositionChangeNotification(*this);
   else
     view()->unregisterMediaForPositionChangeNotification(*this);
 }
 
+LayoutUnit LayoutMedia::computePanelWidth(const LayoutRect& mediaRect) const {
+  FrameHost* frameHost = document().frameHost();
+  LocalFrame* mainFrame = document().page()->deprecatedLocalMainFrame();
+  FrameView* pageView = mainFrame ? mainFrame->view() : nullptr;
+  if (!frameHost || !mainFrame || !pageView)
+    return mediaRect.width();
+
+  if (pageView->horizontalScrollbarMode() != ScrollbarAlwaysOff)
+    return mediaRect.width();
+
+  // On desktop, this will include scrollbars when they stay visible.
+  const LayoutUnit visibleWidth(frameHost->visualViewport().visibleWidth());
+  const LayoutUnit absoluteXOffset(
+      localToAbsolute(
+          FloatPoint(mediaRect.location()),
+          UseTransforms | ApplyContainerFlip | TraverseDocumentBoundaries)
+          .x());
+  DCHECK_GE(visibleWidth - absoluteXOffset, 0);
+
+  return std::min(mediaRect.width(), visibleWidth - absoluteXOffset);
+}
+
 }  // namespace blink