Bug 1656802. Add state variables to the scroll frame to track when scrollbars are only created to scroll the visual viewport within the layout viewport. r=emilio,kats

We need to distinguish these special scrollbars for several different reasons in upcoming patches.

Differential Revision: https://phabricator.services.mozilla.com/D85702

layout/generic/nsGfxScrollFrame.cpp

@@ -323,6 +323,13 @@ struct MOZ_STACK_CLASS ScrollReflowInput {
   // Whether we decided to show the vertical scrollbar
   MOZ_INIT_OUTSIDE_CTOR
   bool mShowVScrollbar;
+  // If mShow(H|V)Scrollbar is true then
+  // mOnlyNeed(V|H)ScrollbarToScrollVVInsideLV indicates if the only reason we
+  // need that scrollbar is to scroll the visual viewport inside the layout
+  // viewport. These scrollbars are special in that even if they are layout
+  // scrollbars they do not take up any layout space.
+  bool mOnlyNeedHScrollbarToScrollVVInsideLV = false;
+  bool mOnlyNeedVScrollbarToScrollVVInsideLV = false;
 
   ScrollReflowInput(nsIScrollableFrame* aFrame, const ReflowInput& aReflowInput)
       : mReflowInput(aReflowInput),
@@ -602,6 +609,7 @@ bool nsHTMLScrollFrame::TryLayout(ScrollReflowInput* aState,
           vvChanged = true;
           visualViewportSize.height -= hScrollbarPrefSize.height;
           aState->mShowHScrollbar = true;
+          aState->mOnlyNeedHScrollbarToScrollVVInsideLV = true;
           ROOT_SCROLLBAR_LOG("TryLayout added H scrollbar for VV, VV now %s\n",
                              Stringify(visualViewportSize).c_str());
         }
@@ -615,6 +623,7 @@ bool nsHTMLScrollFrame::TryLayout(ScrollReflowInput* aState,
           vvChanged = true;
           visualViewportSize.width -= vScrollbarPrefSize.width;
           aState->mShowVScrollbar = true;
+          aState->mOnlyNeedVScrollbarToScrollVVInsideLV = true;
           ROOT_SCROLLBAR_LOG("TryLayout added V scrollbar for VV, VV now %s\n",
                              Stringify(visualViewportSize).c_str());
         }
@@ -801,7 +810,12 @@ bool nsHTMLScrollFrame::GuessHScrollbarNeeded(const ScrollReflowInput& aState) {
     // no guessing required
     return aState.mHScrollbar == ShowScrollbar::Always;
   }
-  return mHelper.mHasHorizontalScrollbar;
+  // We only care about scrollbars that might take up space when trying to guess
+  // if we need a scrollbar, so we ignore scrollbars only created to scroll the
+  // visual viewport inside the layout viewport because they take up no layout
+  // space.
+  return mHelper.mHasHorizontalScrollbar &&
+         !mHelper.mOnlyNeedHScrollbarToScrollVVInsideLV;
 }
 
 bool nsHTMLScrollFrame::GuessVScrollbarNeeded(const ScrollReflowInput& aState) {
@@ -814,7 +828,12 @@ bool nsHTMLScrollFrame::GuessVScrollbarNeeded(const ScrollReflowInput& aState) {
   // the state of the vertical scrollbar will be what we determined
   // last time.
   if (mHelper.mHadNonInitialReflow) {
-    return mHelper.mHasVerticalScrollbar;
+    // We only care about scrollbars that might take up space when trying to
+    // guess if we need a scrollbar, so we ignore scrollbars only created to
+    // scroll the visual viewport inside the layout viewport because they take
+    // up no layout space.
+    return mHelper.mHasVerticalScrollbar &&
+           !mHelper.mOnlyNeedVScrollbarToScrollVVInsideLV;
   }
 
   // If this is the initial reflow, guess false because usually
@@ -1277,6 +1296,13 @@ void nsHTMLScrollFrame::Reflow(nsPresContext* aPresContext,
     mHelper.mPostedReflowCallback = true;
   }
 
+  bool didOnlyHScrollbar = mHelper.mOnlyNeedHScrollbarToScrollVVInsideLV;
+  bool didOnlyVScrollbar = mHelper.mOnlyNeedVScrollbarToScrollVVInsideLV;
+  mHelper.mOnlyNeedHScrollbarToScrollVVInsideLV =
+      state.mOnlyNeedHScrollbarToScrollVVInsideLV;
+  mHelper.mOnlyNeedVScrollbarToScrollVVInsideLV =
+      state.mOnlyNeedVScrollbarToScrollVVInsideLV;
+
   bool didHaveHScrollbar = mHelper.mHasHorizontalScrollbar;
   bool didHaveVScrollbar = mHelper.mHasVerticalScrollbar;
   mHelper.mHasHorizontalScrollbar = state.mShowHScrollbar;
@@ -1288,6 +1314,8 @@ void nsHTMLScrollFrame::Reflow(nsPresContext* aPresContext,
       reflowScrollCorner || HasAnyStateBits(NS_FRAME_IS_DIRTY) ||
       didHaveHScrollbar != state.mShowHScrollbar ||
       didHaveVScrollbar != state.mShowVScrollbar ||
+      didOnlyHScrollbar != mHelper.mOnlyNeedHScrollbarToScrollVVInsideLV ||
+      didOnlyVScrollbar != mHelper.mOnlyNeedVScrollbarToScrollVVInsideLV ||
       !oldScrollAreaBounds.IsEqualEdges(newScrollAreaBounds) ||
       !oldScrolledAreaBounds.IsEqualEdges(newScrolledAreaBounds)) {
     if (!mHelper.mSuppressScrollbarUpdate) {
@@ -2214,6 +2242,8 @@ ScrollFrameHelper::ScrollFrameHelper(nsContainerFrame* aOuter, bool aIsRoot)
       mNeverHasHorizontalScrollbar(false),
       mHasVerticalScrollbar(false),
       mHasHorizontalScrollbar(false),
+      mOnlyNeedVScrollbarToScrollVVInsideLV(false),
+      mOnlyNeedHScrollbarToScrollVVInsideLV(false),
       mFrameIsUpdatingScrollbar(false),
       mDidHistoryRestore(false),
       mIsRoot(aIsRoot),

layout/generic/nsGfxScrollFrame.h

@@ -630,6 +630,13 @@ class ScrollFrameHelper : public nsIReflowCallback {
   bool mNeverHasHorizontalScrollbar : 1;
   bool mHasVerticalScrollbar : 1;
   bool mHasHorizontalScrollbar : 1;
+  // If mHas(Vertical|Horizontal)Scrollbar is true then
+  // mOnlyNeed(V|H)ScrollbarToScrollVVInsideLV indicates if the only reason we
+  // need that scrollbar is to scroll the visual viewport inside the layout
+  // viewport. These scrollbars are special in that even if they are layout
+  // scrollbars they do not take up any layout space.
+  bool mOnlyNeedVScrollbarToScrollVVInsideLV : 1;
+  bool mOnlyNeedHScrollbarToScrollVVInsideLV : 1;
   bool mFrameIsUpdatingScrollbar : 1;
   bool mDidHistoryRestore : 1;
   // Is this the scrollframe for the document's viewport?