Bug 1758564 - Allow rendering children of <img>'s shadow root. r=dholbert

This doesn't change behavior just yet, but seems worth doing separately.

With this patch, we can render content overlaid on top of an image
frame, by attaching a shadow root to it. The idea is to use this for
text recognition (OCR) of text inside images.

I chose to implement this using the DynamicLeaf stuff that I removed in
bug 1746310 (because nsMenuPopupFrame was its only user).

That seems simpler than either a new frame class, or more special cases
in the frame constructor, etc. But let me know if you disagree.

There are further changes that we want to do. For example, trying to
select the overlaid text with the mouse right now will start dragging
the image. For that, we might need to tweak our selection / mouse
dragging code. But those seem all changes that can go on top of this.

Differential Revision: https://phabricator.services.mozilla.com/D140638

layout/base/nsCSSFrameConstructor.cpp

@@ -1681,6 +1681,14 @@ void nsCSSFrameConstructor::CreateGeneratedContentFromListStyleType(
   aAddChild(child);
 }
 
+// Frames for these may not be leaves in the proper sense, but we still don't
+// want to expose generated content on them. For the purposes of the page they
+// should be leaves.
+static bool HasUAWidget(const Element& aOriginatingElement) {
+  const ShadowRoot* sr = aOriginatingElement.GetShadowRoot();
+  return sr && sr->IsUAWidget();
+}
+
 /*
  * aParentFrame - the frame that should be the parent of the generated
  *   content.  This is the frame for the corresponding content node,
@@ -1705,10 +1713,7 @@ void nsCSSFrameConstructor::CreateGeneratedContentItem(
                  aPseudoElement == PseudoStyleType::marker,
              "unexpected aPseudoElement");
 
-  if (aParentFrame && (aParentFrame->IsHTMLVideoFrame() ||
-                       aParentFrame->IsDateTimeControlFrame())) {
-    // Video frames and date time control frames may not be leafs when backed by
-    // an UA widget, but we still don't want to expose generated content.
+  if (HasUAWidget(aOriginatingElement)) {
     return;
   }
 

layout/generic/FrameClass.py

@@ -6,6 +6,7 @@
 # Leaf constants to pass to Frame's leafness argument.
 LEAF = "Leaf"
 NOT_LEAF = "NotLeaf"
+DYNAMIC_LEAF = "DynamicLeaf"
 
 
 class FrameClass:

layout/generic/FrameClasses.py

@@ -3,7 +3,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 # Frame class definitions, used to generate FrameIdList.h and FrameTypeList.h
 
-from FrameClass import Frame, AbstractFrame, LEAF, NOT_LEAF
+from FrameClass import Frame, AbstractFrame, LEAF, NOT_LEAF, DYNAMIC_LEAF
 
 FRAME_CLASSES = [
     Frame("BRFrame", "Br", LEAF),
@@ -41,7 +41,7 @@ FRAME_CLASSES = [
     Frame("nsHTMLScrollFrame", "Scroll", NOT_LEAF),
     Frame("nsImageBoxFrame", "ImageBox", LEAF),
     Frame("nsImageControlFrame", "ImageControl", LEAF),
-    Frame("nsImageFrame", "Image", LEAF),
+    Frame("nsImageFrame", "Image", DYNAMIC_LEAF),
     Frame("nsInlineFrame", "Inline", NOT_LEAF),
     Frame("nsLeafBoxFrame", "LeafBox", LEAF),
     Frame("nsListControlFrame", "ListControl", NOT_LEAF),

layout/generic/nsIFrame.cpp

@@ -171,11 +171,13 @@ const nsIFrame::FrameClassBits nsIFrame::sFrameClassBits[
     0] = {
 #define Leaf eFrameClassBitsLeaf
 #define NotLeaf eFrameClassBitsNone
+#define DynamicLeaf eFrameClassBitsDynamicLeaf
 #define FRAME_ID(class_, type_, leaf_, ...) leaf_,
 #define ABSTRACT_FRAME_ID(...)
 #include "mozilla/FrameIdList.h"
 #undef Leaf
 #undef NotLeaf
+#undef DynamicLeaf
 #undef FRAME_ID
 #undef ABSTRACT_FRAME_ID
 };

layout/generic/nsIFrame.h

@@ -3366,9 +3366,14 @@ class nsIFrame : public nsQueryFrame {
   bool IsLeaf() const {
     MOZ_ASSERT(uint8_t(mClass) < mozilla::ArrayLength(sFrameClassBits));
     FrameClassBits bits = sFrameClassBits[uint8_t(mClass)];
+    if (MOZ_UNLIKELY(bits & eFrameClassBitsDynamicLeaf)) {
+      return IsLeafDynamic();
+    }
     return bits & eFrameClassBitsLeaf;
   }
 
+  virtual bool IsLeafDynamic() const { return false; }
+
   /**
    * Marks all display items created by this frame as needing a repaint,
    * and calls SchedulePaint() if requested and one is not already pending.
@@ -5413,6 +5418,7 @@ class nsIFrame : public nsQueryFrame {
   enum FrameClassBits {
     eFrameClassBitsNone = 0x0,
     eFrameClassBitsLeaf = 0x1,
+    eFrameClassBitsDynamicLeaf = 0x2,
   };
   // Maps mClass to IsLeaf() flags.
   static const FrameClassBits sFrameClassBits[

layout/generic/nsImageFrame.cpp

@@ -1209,6 +1209,31 @@ nscoord nsImageFrame::GetPrefISize(gfxContext* aRenderingContext) {
   return iSize.valueOr(0);
 }
 
+void nsImageFrame::ReflowChildren(nsPresContext* aPresContext,
+                                  const ReflowInput& aReflowInput,
+                                  const LogicalSize& aImageSize) {
+  for (nsIFrame* child : mFrames) {
+    ReflowOutput childDesiredSize(aReflowInput);
+    WritingMode wm = GetWritingMode();
+    // Shouldn't be hard to support if we want, but why bother.
+    MOZ_ASSERT(
+        wm == child->GetWritingMode(),
+        "We don't expect mismatched writing-modes in content we control");
+    nsReflowStatus childStatus;
+
+    LogicalPoint childOffset(wm);
+    ReflowInput childReflowInput(aPresContext, aReflowInput, child, aImageSize);
+    const nsSize containerSize = aImageSize.GetPhysicalSize(wm);
+    ReflowChild(child, aPresContext, childDesiredSize, childReflowInput, wm,
+                childOffset, containerSize, ReflowChildFlags::Default,
+                childStatus);
+
+    FinishReflowChild(child, aPresContext, childDesiredSize, &childReflowInput,
+                      wm, childOffset, containerSize,
+                      ReflowChildFlags::Default);
+  }
+}
+
 void nsImageFrame::Reflow(nsPresContext* aPresContext, ReflowOutput& aMetrics,
                           const ReflowInput& aReflowInput,
                           nsReflowStatus& aStatus) {
@@ -1296,6 +1321,9 @@ void nsImageFrame::Reflow(nsPresContext* aPresContext, ReflowOutput& aMetrics,
   }
   FinishAndStoreOverflow(&aMetrics, aReflowInput.mStyleDisplay);
 
+  // Reflow the child frames. Our children can't affect our size in any way.
+  ReflowChildren(aPresContext, aReflowInput, aMetrics.Size(GetWritingMode()));
+
   if (HasAnyStateBits(NS_FRAME_FIRST_REFLOW) && !mReflowCallbackPosted) {
     mReflowCallbackPosted = true;
     PresShell()->PostReflowCallback(this);
@@ -2326,6 +2354,8 @@ void nsImageFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
     DisplaySelectionOverlay(aBuilder, aLists.Content(),
                             nsISelectionDisplay::DISPLAY_IMAGES);
   }
+
+  BuildDisplayListForNonBlockChildren(aBuilder, aLists);
 }
 
 bool nsImageFrame::ShouldDisplaySelection() {
@@ -2400,6 +2430,19 @@ bool nsImageFrame::GetAnchorHREFTargetAndNode(nsIURI** aHref, nsString& aTarget,
   return status;
 }
 
+bool nsImageFrame::IsLeafDynamic() const {
+  if (mKind != Kind::ImageElement) {
+    // Image frames created for "content: url()" could have an author-controlled
+    // shadow root, we want to be a regular leaf for those.
+    return true;
+  }
+  // For elements that create image frames, calling attachShadow() will throw,
+  // so the only ShadowRoot we could ever have is a UA widget.
+  const auto* shadow = mContent->AsElement()->GetShadowRoot();
+  MOZ_ASSERT_IF(shadow, shadow->IsUAWidget());
+  return !shadow;
+}
+
 nsresult nsImageFrame::GetContentForEvent(WidgetEvent* aEvent,
                                           nsIContent** aContent) {
   NS_ENSURE_ARG_POINTER(aContent);

layout/generic/nsImageFrame.h

@@ -84,6 +84,7 @@ class nsImageFrame : public nsAtomicContainerFrame, public nsIReflowCallback {
   }
   void Reflow(nsPresContext*, ReflowOutput&, const ReflowInput&,
               nsReflowStatus&) override;
+  bool IsLeafDynamic() const override;
 
   nsresult GetContentForEvent(mozilla::WidgetEvent*,
                               nsIContent** aContent) final;
@@ -212,6 +213,9 @@ class nsImageFrame : public nsAtomicContainerFrame, public nsIReflowCallback {
 
   nsImageFrame(ComputedStyle*, nsPresContext* aPresContext, ClassID, Kind);
 
+  void ReflowChildren(nsPresContext*, const ReflowInput&,
+                      const mozilla::LogicalSize& aImageSize);
+
  protected:
   nsImageFrame(ComputedStyle* aStyle, nsPresContext* aPresContext, ClassID aID)
       : nsImageFrame(aStyle, aPresContext, aID, Kind::ImageElement) {}