Bug 624647 part 0: Don't use ASSUME_DRAWING_RESTRICTED_TO_CONTENT_RECT flag if 'object-fit' and/or 'object-position' might make our drawing overflow. r=roc

layout/generic/nsImageFrame.cpp

@@ -31,6 +31,7 @@
 #include "nsStyleContext.h"
 #include "nsStyleConsts.h"
 #include "nsStyleCoord.h"
+#include "nsStyleUtil.h"
 #include "nsTransform2D.h"
 #include "nsImageMap.h"
 #include "nsIIOService.h"
@@ -1528,8 +1529,12 @@ nsImageFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
 
   DisplayBorderBackgroundOutline(aBuilder, aLists);
 
+  uint32_t clipFlags =
+    nsStyleUtil::ObjectPropsMightCauseOverflow(StylePosition()) ?
+    0 : DisplayListClipState::ASSUME_DRAWING_RESTRICTED_TO_CONTENT_RECT;
+
   DisplayListClipState::AutoClipContainingBlockDescendantsToContentBox
-    clip(aBuilder, this, DisplayListClipState::ASSUME_DRAWING_RESTRICTED_TO_CONTENT_RECT);
+    clip(aBuilder, this, clipFlags);
 
   if (mComputedSize.width != 0 && mComputedSize.height != 0) {
     nsCOMPtr<nsIImageLoadingContent> imageLoader = do_QueryInterface(mContent);

layout/generic/nsVideoFrame.cpp

@@ -25,6 +25,7 @@
 #include "ImageContainer.h"
 #include "ImageLayers.h"
 #include "nsContentList.h"
+#include "nsStyleUtil.h"
 #include <algorithm>
 
 using namespace mozilla;
@@ -432,10 +433,24 @@ nsVideoFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
 
   DisplayBorderBackgroundOutline(aBuilder, aLists);
 
-  DisplayListClipState::AutoClipContainingBlockDescendantsToContentBox
-    clip(aBuilder, this, DisplayListClipState::ASSUME_DRAWING_RESTRICTED_TO_CONTENT_RECT);
+  const bool shouldDisplayPoster = ShouldDisplayPoster();
 
-  if (HasVideoElement() && !ShouldDisplayPoster()) {
+  // NOTE: If we're displaying a poster image (instead of video data), we can
+  // trust the nsImageFrame to constrain its drawing to its content rect
+  // (which happens to be the same as our content rect).
+  uint32_t clipFlags;
+  if (shouldDisplayPoster ||
+      !nsStyleUtil::ObjectPropsMightCauseOverflow(StylePosition())) {
+    clipFlags =
+      DisplayListClipState::ASSUME_DRAWING_RESTRICTED_TO_CONTENT_RECT;
+  } else {
+    clipFlags = 0;
+  }
+
+  DisplayListClipState::AutoClipContainingBlockDescendantsToContentBox
+    clip(aBuilder, this, clipFlags);
+
+  if (HasVideoElement() && !shouldDisplayPoster) {
     aLists.Content()->AppendNewToTop(
       new (aBuilder) nsDisplayVideo(aBuilder, this));
   }
@@ -446,7 +461,7 @@ nsVideoFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   for (nsIFrame *child = mFrames.FirstChild();
        child;
        child = child->GetNextSibling()) {
-    if (child->GetContent() != mPosterImage || ShouldDisplayPoster()) {
+    if (child->GetContent() != mPosterImage || shouldDisplayPoster) {
       child->BuildDisplayListForStackingContext(aBuilder,
                                                 aDirtyRect - child->GetOffsetTo(this),
                                                 aLists.Content());

layout/style/nsStyleUtil.cpp

@@ -10,6 +10,7 @@
 #include "nsCSSProps.h"
 #include "nsRuleNode.h"
 #include "nsROCSSPrimitiveValue.h"
+#include "nsStyleStruct.h"
 #include "nsIContentPolicy.h"
 #include "nsIContentSecurityPolicy.h"
 #include "nsIURI.h"
@@ -619,6 +620,57 @@ nsStyleUtil::IsFlexBasisMainSize(const nsStyleCoord& aFlexBasis,
   return aFlexBasis.GetIntValue() == NS_STYLE_FLEX_BASIS_MAIN_SIZE;
 }
 
+// For a replaced element whose concrete object size is no larger than the
+// element's content-box, this method checks whether the given
+// "object-position" coordinate might cause overflow in its dimension.
+typedef nsStyleBackground::Position::PositionCoord PositionCoord;
+static bool
+ObjectPositionCoordMightCauseOverflow(const PositionCoord& aCoord)
+{
+  // Any nonzero length in "object-position" can push us to overflow
+  // (particularly if our concrete object size is exactly the same size as the
+  // replaced element's content-box).
+  if (aCoord.mLength != 0) {
+    return true;
+  }
+
+  // Percentages are interpreted as a fraction of the extra space. So,
+  // percentages in the 0-100% range are safe, but values outside of that
+  // range could cause overflow.
+  if (aCoord.mHasPercent &&
+      (aCoord.mPercent < 0.0f || aCoord.mPercent > 1.0f)) {
+    return true;
+  }
+  return false;
+}
+
+
+/* static */ bool
+nsStyleUtil::ObjectPropsMightCauseOverflow(const nsStylePosition* aStylePos)
+{
+  auto objectFit = aStylePos->mObjectFit;
+
+  // "object-fit: cover" & "object-fit: none" can give us a render rect that's
+  // larger than our container element's content-box.
+  if (objectFit == NS_STYLE_OBJECT_FIT_COVER ||
+      objectFit == NS_STYLE_OBJECT_FIT_NONE) {
+    return true;
+  }
+  // (All other object-fit values produce a concrete object size that's no larger
+  // than the constraint region.)
+
+  // Check each of our "object-position" coords to see if it could cause
+  // overflow in its dimension:
+  const nsStyleBackground::Position& objectPosistion = aStylePos->mObjectPosition;
+  if (ObjectPositionCoordMightCauseOverflow(objectPosistion.mXPosition) ||
+      ObjectPositionCoordMightCauseOverflow(objectPosistion.mYPosition)) {
+    return true;
+  }
+
+  return false;
+}
+
+
 /* static */ bool
 nsStyleUtil::CSPAllowsInlineStyle(nsIContent* aContent,
                                   nsIPrincipal* aPrincipal,

layout/style/nsStyleUtil.h

@@ -20,6 +20,7 @@ class nsIURI;
 struct gfxFontFeature;
 struct gfxAlternateValue;
 struct nsCSSValueList;
+struct nsStylePosition;
 
 // Style utility functions
 class nsStyleUtil {
@@ -130,6 +131,23 @@ public:
   static bool IsFlexBasisMainSize(const nsStyleCoord& aFlexBasis,
                                   bool aIsMainAxisHorizontal);
 
+  /**
+   * Returns true if our object-fit & object-position properties might cause
+   * a replaced element's contents to overflow its content-box (requiring
+   * clipping), or false if we can be sure that this won't happen.
+   *
+   * This lets us optimize by skipping clipping when we can tell it's
+   * unnecessary (particularly with the default values of these properties).
+   *
+   * @param aStylePos The nsStylePosition whose object-fit & object-position
+   *                  properties should be checked for potential overflow.
+   * @return false if we can be sure that the object-fit & object-position
+   *         properties on 'aStylePos' cannot cause a replaced element's
+   *         contents to overflow its content-box. Otherwise (if overflow is
+   *         is possible), returns true.
+   */
+  static bool ObjectPropsMightCauseOverflow(const nsStylePosition* aStylePos);
+
   /*
    *  Does this principal have a CSP that blocks the application of
    *  inline styles? Returns false if application of the style should