Bug 1897135 Part 3 - Change two APIs to ensure root frame is a ViewportFrame. r=dholbert

Move `SetRootFrame()` to nsFrameManager.cpp, because in order to compile
`mRootFrame = aRootFrame` the compiler requires the full definition of
`ViewportFrame` to know that `ViewportFrame` is a subclass of `nsIFrame`

Differential Revision: https://phabricator.services.mozilla.com/D210671

layout/base/PresShell.cpp

@@ -115,6 +115,7 @@
 #include "mozilla/Try.h"
 #include "mozilla/UniquePtr.h"
 #include "mozilla/Unused.h"
+#include "mozilla/ViewportFrame.h"
 #include "mozilla/ViewportUtils.h"
 #include "nsAnimationManager.h"
 #include "nsAutoLayoutPhase.h"
@@ -1766,10 +1767,10 @@ nsresult PresShell::Initialize() {
   // XXXbz it would be nice to move this somewhere else... like frame manager
   // Init(), say.  But we need to make sure our views are all set up by the
   // time we do this!
-  nsIFrame* rootFrame = mFrameConstructor->GetRootFrame();
-  NS_ASSERTION(!rootFrame, "How did that happen, exactly?");
-
-  if (!rootFrame) {
+  MOZ_ASSERT(!mFrameConstructor->GetRootFrame(),
+             "How did that happen, exactly?");
+  ViewportFrame* rootFrame;
+  {
     nsAutoScriptBlocker scriptBlocker;
     rootFrame = mFrameConstructor->ConstructRootFrame();
     mFrameConstructor->SetRootFrame(rootFrame);
@@ -2458,8 +2459,7 @@ nsIFrame* PresShell::GetRootScrollFrame() const {
     return nullptr;
   }
   nsIFrame* rootFrame = mFrameConstructor->GetRootFrame();
-  // Ensure root frame is a viewport frame
-  if (!rootFrame || !rootFrame->IsViewportFrame()) {
+  if (!rootFrame) {
     return nullptr;
   }
   nsIFrame* theFrame = rootFrame->PrincipalChildList().FirstChild();

layout/base/nsCSSFrameConstructor.cpp

@@ -2635,7 +2635,7 @@ RestyleManager* nsCSSFrameConstructor::RestyleManager() const {
   return mPresShell->GetPresContext()->RestyleManager();
 }
 
-nsIFrame* nsCSSFrameConstructor::ConstructRootFrame() {
+ViewportFrame* nsCSSFrameConstructor::ConstructRootFrame() {
   AUTO_PROFILER_LABEL_HOT("nsCSSFrameConstructor::ConstructRootFrame",
                           LAYOUT_FrameConstruction);
   AUTO_LAYOUT_PHASE_ENTRY_POINT(mPresShell->GetPresContext(), FrameC);

layout/base/nsCSSFrameConstructor.h

@@ -45,6 +45,7 @@ class ComputedStyle;
 class PresShell;
 class PrintedSheetFrame;
 class RestyleManager;
+class ViewportFrame;
 
 namespace dom {
 
@@ -96,7 +97,7 @@ class nsCSSFrameConstructor final : public nsFrameManager {
 
   mozilla::RestyleManager* RestyleManager() const;
 
-  nsIFrame* ConstructRootFrame();
+  mozilla::ViewportFrame* ConstructRootFrame();
 
  private:
   enum Operation { CONTENTAPPEND, CONTENTINSERT };

layout/base/nsFrameManager.cpp

@@ -16,6 +16,7 @@
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/PresShell.h"
 #include "mozilla/PresState.h"
+#include "mozilla/ViewportFrame.h"
 #include "nsAbsoluteContainingBlock.h"
 #include "nsCOMPtr.h"
 #include "nsContainerFrame.h"
@@ -37,6 +38,12 @@ nsFrameManager::~nsFrameManager() {
   NS_ASSERTION(!mPresShell, "nsFrameManager::Destroy never called");
 }
 
+void nsFrameManager::SetRootFrame(ViewportFrame* aRootFrame) {
+  MOZ_ASSERT(aRootFrame, "The root frame should be valid!");
+  MOZ_ASSERT(!mRootFrame, "We should set a root frame only once!");
+  mRootFrame = aRootFrame;
+}
+
 void nsFrameManager::Destroy() {
   NS_ASSERTION(mPresShell, "Frame manager already shut down.");
 

layout/base/nsFrameManager.h

@@ -22,6 +22,7 @@ class nsWindowSizes;
 namespace mozilla {
 struct FrameDestroyContext;
 class PresShell;
+class ViewportFrame;
 }  // namespace mozilla
 
 /**
@@ -35,21 +36,18 @@ class nsFrameManager {
   using DestroyContext = mozilla::FrameDestroyContext;
 
   explicit nsFrameManager(mozilla::PresShell* aPresShell)
-      : mPresShell(aPresShell), mRootFrame(nullptr) {
+      : mPresShell(aPresShell) {
     MOZ_ASSERT(mPresShell, "need a pres shell");
   }
   ~nsFrameManager();
 
   /*
-   * Gets and sets the root frame (typically the viewport). The lifetime of the
+   * Gets and sets the root frame (i.e. ViewportFrame). The lifetime of the
    * root frame is controlled by the frame manager. When the frame manager is
    * destroyed, it destroys the entire frame hierarchy.
    */
   nsIFrame* GetRootFrame() const { return mRootFrame; }
-  void SetRootFrame(nsIFrame* aRootFrame) {
-    NS_ASSERTION(!mRootFrame, "already have a root frame");
-    mRootFrame = aRootFrame;
-  }
+  void SetRootFrame(mozilla::ViewportFrame* aRootFrame);
 
   /*
    * After Destroy is called, it is an error to call any FrameManager methods.
@@ -94,7 +92,11 @@ class nsFrameManager {
  protected:
   // weak link, because the pres shell owns us
   mozilla::PresShell* MOZ_NON_OWNING_REF mPresShell;
-  nsIFrame* mRootFrame;
+
+  // Note: We use nsIFrame* instead of ViewportFrame* for mRootFrame to avoid
+  // exposing ViewportFrame to the callers via GetRootFrame(), since they do not
+  // depend on any ViewportFrame-specific APIs or details.
+  nsIFrame* mRootFrame = nullptr;
 };
 
 #endif

layout/generic/ViewportFrame.h

@@ -6,7 +6,7 @@
 
 /*
  * rendering object that is the root of the frame tree, which contains
- * the document's scrollbars and contains fixed-positioned elements
+ * contains fixed-positioned elements
  */
 
 #ifndef mozilla_ViewportFrame_h
@@ -23,9 +23,10 @@ class nsDisplayWrapList;
 class ServoRestyleState;
 
 /**
- * ViewportFrame is the parent of a single child - the doc root frame or a
- * scroll frame containing the doc root frame. ViewportFrame stores this child
- * in its primary child list.
+ * ViewportFrame is the parent of a single child -- the root canvas frame or a
+ * scroll container frame containing the root canvas frame. See
+ * nsCSSFrameConstructor::SetUpDocElementContainingBlock() for the root frame
+ * hierarchy. ViewportFrame stores this child in its primary child list.
  */
 class ViewportFrame : public nsContainerFrame {
  public: