Bug 1665476 - Remove nsRootBoxFrame to unify background propagation between XUL and non-XUL documents. r=layout-reviewers,mats

It's unused on mozilla-central, and Thunderbird can just use the canvas
frame as regular (X)HTML documents, so just use a canvas frame instead
of an nsRootBoxFrame for XUL as well.

nsRootBoxFrame was needed because of various XUL-specific things like
tooltips and so on lived there. But with the move away from XUL, that
functionality has been added to nsCanvasFrame already, behind a
principal check instead.

This also allows simplifying our background propagation setup, which was
only half-working for XUL documents (this bug is a consequence of that).

With this, most of the callers of nsCSSRendering::IsCanvasFrame can go.
They're only two of the frames that would return true for that that
actually paint backgrounds (nsCanvasFrame and nsRootBoxFrame), so the
codepaths in display list building and painting can just check
frame->IsCanvasFrame() instead.

The remaining caller to that function is
nsContainerFrame::SyncWindowProperties, and the change is also legit, in
the sense that the only thing SyncWindowProperties() really cares about
is propagating the max/min-width constraints from the root element's
style to the view/widget, and the only frame that would return true from
IsCanvasFrame and have a view is the viewport frame which is the root of
the frame tree.

Differential Revision: https://phabricator.services.mozilla.com/D90846

layout/base/nsCSSFrameConstructor.cpp

@@ -209,9 +209,6 @@ static FrameCtorDebugFlags gFlags[] = {
 
 //------------------------------------------------------------------
 
-nsContainerFrame* NS_NewRootBoxFrame(PresShell* aPresShell,
-                                     ComputedStyle* aStyle);
-
 nsContainerFrame* NS_NewDocElementBoxFrame(PresShell* aPresShell,
                                            ComputedStyle* aStyle);
 
@@ -1467,7 +1464,6 @@ nsCSSFrameConstructor::nsCSSFrameConstructor(Document* aDocument,
       mQuotesDirty(false),
       mCountersDirty(false),
       mIsDestroyingFrameTree(false),
-      mHasRootAbsPosContainingBlock(false),
       mAlwaysCreateFramesForIgnorableWhitespace(false) {
 #ifdef DEBUG
   static bool gFirstTime = true;
@@ -2397,14 +2393,12 @@ nsIFrame* nsCSSFrameConstructor::ConstructDocElementFrame(
   }
 
   nsFrameConstructorSaveState docElementContainingBlockAbsoluteSaveState;
-  if (mHasRootAbsPosContainingBlock) {
-    // Push the absolute containing block now so we can absolutely position
-    // the root element
-    mDocElementContainingBlock->AddStateBits(NS_FRAME_CAN_HAVE_ABSPOS_CHILDREN);
-    state.PushAbsoluteContainingBlock(
-        mDocElementContainingBlock, mDocElementContainingBlock,
-        docElementContainingBlockAbsoluteSaveState);
-  }
+  // Push the absolute containing block now so we can absolutely position
+  // the root element
+  mDocElementContainingBlock->AddStateBits(NS_FRAME_CAN_HAVE_ABSPOS_CHILDREN);
+  state.PushAbsoluteContainingBlock(
+      mDocElementContainingBlock, mDocElementContainingBlock,
+      docElementContainingBlockAbsoluteSaveState);
 
   // The rules from CSS 2.1, section 9.2.4, have already been applied
   // by the style system, so we can assume that display->mDisplay is
@@ -2619,7 +2613,7 @@ void nsCSSFrameConstructor::SetUpDocElementContainingBlock(
   Galley presentation, XUL
 
       ViewportFrame [fixed-cb]
-        nsRootBoxFrame
+        nsCanvasFrame [abs-cb]
           root element frame (nsDocElementBoxFrame)
 
   Print presentation, non-XUL
@@ -2656,7 +2650,7 @@ void nsCSSFrameConstructor::SetUpDocElementContainingBlock(
     mRootElementFrame is "root element frame".  This is the primary frame for
       the root element.
     mDocElementContainingBlock is the parent of mRootElementFrame
-      (i.e. nsCanvasFrame or nsRootBoxFrame)
+      (i.e. nsCanvasFrame)
     mPageSequenceFrame is the nsPageSequenceFrame, or null if there isn't
       one
   */
@@ -2699,17 +2693,8 @@ void nsCSSFrameConstructor::SetUpDocElementContainingBlock(
       static_cast<nsContainerFrame*>(GetRootFrame());
   ComputedStyle* viewportPseudoStyle = viewportFrame->Style();
 
-  nsContainerFrame* rootFrame = nullptr;
-
-  if (aDocElement->IsXULElement()) {
-    // pass a temporary stylecontext, the correct one will be set later
-    rootFrame = NS_NewRootBoxFrame(mPresShell, viewportPseudoStyle);
-  } else {
-    // pass a temporary stylecontext, the correct one will be set later
-    rootFrame = NS_NewCanvasFrame(mPresShell, viewportPseudoStyle);
-    mHasRootAbsPosContainingBlock = true;
-  }
-
+  nsContainerFrame* rootFrame =
+    NS_NewCanvasFrame(mPresShell, viewportPseudoStyle);
   PseudoStyleType rootPseudo = PseudoStyleType::canvas;
   mDocElementContainingBlock = rootFrame;
 
@@ -2803,7 +2788,6 @@ void nsCSSFrameConstructor::SetUpDocElementContainingBlock(
     // The eventual parent of the document element frame.
     // XXX should this be set for every new page (in ConstructPageFrame)?
     mDocElementContainingBlock = canvasFrame;
-    mHasRootAbsPosContainingBlock = true;
   }
 
   if (viewportFrame->HasAnyStateBits(NS_FRAME_FIRST_REFLOW)) {
@@ -5709,7 +5693,7 @@ nsContainerFrame* nsCSSFrameConstructor::GetAbsoluteContainingBlock(
   // It is possible for the search for the containing block to fail, because
   // no absolute container can be found in the parent chain.  In those cases,
   // we fall back to the document element's containing block.
-  return mHasRootAbsPosContainingBlock ? mDocElementContainingBlock : nullptr;
+  return mDocElementContainingBlock;
 }
 
 nsContainerFrame* nsCSSFrameConstructor::GetFloatContainingBlock(
@@ -7593,7 +7577,6 @@ bool nsCSSFrameConstructor::ContentRemoved(nsIContent* aChild,
       mRootElementStyleFrame = nullptr;
       mDocElementContainingBlock = nullptr;
       mPageSequenceFrame = nullptr;
-      mHasRootAbsPosContainingBlock = false;
     }
 
     if (haveFLS && mRootElementFrame) {