Bug 1824852. Ensure PrintedSheetFrame calls MoveOverflowToChildList before reflow. r=dholbert

The intention is to allow PrintedSheetFrame::Reflow to access the page style
prior to reflowing its child so that it can decide whether to apply page style
to the sheet (in the case of one page-per-sheet). This will happen in a
subsequent bug.

Differential Revision: https://phabricator.services.mozilla.com/D173773

layout/generic/PrintedSheetFrame.cpp

@@ -62,6 +62,10 @@ static bool TagIfSkippedByCustomRange(nsPageFrame* aPageFrame, int32_t aPageNum,
   return true;
 }
 
+void PrintedSheetFrame::ClaimPageFrameFromPrevInFlow() {
+  MoveOverflowToChildList();
+}
+
 void PrintedSheetFrame::Reflow(nsPresContext* aPresContext,
                                ReflowOutput& aReflowOutput,
                                const ReflowInput& aReflowInput,

layout/generic/PrintedSheetFrame.h

@@ -26,6 +26,14 @@ class PrintedSheetFrame final : public nsContainerFrame {
 
   void SetSharedPageData(nsSharedPageData* aPD) { mPD = aPD; }
 
+  // Invokes MoveOverflowToChildList.
+  // This is intended for use by callers that need to be able to get our first/
+  // only nsPageFrame from our child list to examine its computed style just
+  // **prior** to us being reflowed. (If our first nsPageFrame will come from
+  // our prev-in-flow, we won't otherwise take ownership of it until we are
+  // reflowed.)
+  void ClaimPageFrameFromPrevInFlow();
+
   // nsIFrame overrides
   void Reflow(nsPresContext* aPresContext, ReflowOutput& aReflowOutput,
               const ReflowInput& aReflowInput,

layout/generic/nsPageSequenceFrame.cpp

@@ -347,6 +347,10 @@ void nsPageSequenceFrame::Reflow(nsPresContext* aPresContext,
     auto* sheet = static_cast<PrintedSheetFrame*>(kidFrame);
     sheet->SetSharedPageData(mPageData.get());
 
+    // If we want to reliably access the nsPageFrame before reflowing the sheet
+    // frame, we need to call this:
+    sheet->ClaimPageFrameFromPrevInFlow();
+
     // Reflow the sheet
     ReflowInput kidReflowInput(
         aPresContext, aReflowInput, kidFrame,