Bug 1646374 - Add a new log module for scroll restoration. r=tnikkel

Depends on D79885

Differential Revision: https://phabricator.services.mozilla.com/D80038

layout/generic/nsGfxScrollFrame.cpp

@@ -94,6 +94,9 @@
 static mozilla::LazyLogModule sApzPaintSkipLog("apz.paintskip");
 #define PAINT_SKIP_LOG(...) \
   MOZ_LOG(sApzPaintSkipLog, LogLevel::Debug, (__VA_ARGS__))
+static mozilla::LazyLogModule sScrollRestoreLog("scrollrestore");
+#define SCROLLRESTORE_LOG(...) \
+  MOZ_LOG(sScrollRestoreLog, LogLevel::Debug, (__VA_ARGS__))
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -2326,6 +2329,8 @@ void ScrollFrameHelper::ScrollToWithOrigin(
   if (aOrigin != ScrollOrigin::Restore) {
     // If we're doing a non-restore scroll, we don't want to later
     // override it by restoring our saved scroll position.
+    SCROLLRESTORE_LOG("%p: Clearing mRestorePos (cur=%d, dst=%d)\n", this,
+                      GetScrollPosition().y, aScrollPosition.y);
     mRestorePos.x = mRestorePos.y = -1;
   }
 
@@ -4673,6 +4678,13 @@ void ScrollFrameHelper::ScrollToRestoredPosition() {
   // 8).
   nsPoint logicalLayoutScrollPos = GetLogicalScrollPosition();
 
+  SCROLLRESTORE_LOG(
+      "%p: ScrollToRestoredPosition (mRestorePos.y=%d, mLastPos.y=%d, "
+      "layoutRestorePos.y=%d, logicalLayoutScrollPos.y=%d, "
+      "GetLogicalVisualViewportOffset().y=%d)\n",
+      this, mRestorePos.y, mLastPos.y, layoutRestorePos.y,
+      logicalLayoutScrollPos.y, GetLogicalVisualViewportOffset().y);
+
   // if we didn't move, we still need to restore
   if (GetLogicalVisualViewportOffset() == mLastPos ||
       logicalLayoutScrollPos == mLastPos) {
@@ -6677,6 +6689,8 @@ UniquePtr<PresState> ScrollFrameHelper::SaveState() const {
     pt = mDestination;
     allowScrollOriginDowngrade = false;
   }
+  SCROLLRESTORE_LOG("%p: SaveState, pt.y=%d, mLastPos.y=%d, mRestorePos.y=%d\n",
+                    this, pt.y, mLastPos.y, mRestorePos.y);
   if (mRestorePos.y != -1 && pt == mLastPos) {
     pt = mRestorePos;
   }
@@ -6705,6 +6719,8 @@ void ScrollFrameHelper::RestoreState(PresState* aState) {
   mLastScrollOrigin = ScrollOrigin::Other;
   mDidHistoryRestore = true;
   mLastPos = mScrolledFrame ? GetLogicalVisualViewportOffset() : nsPoint(0, 0);
+  SCROLLRESTORE_LOG("%p: RestoreState, set mRestorePos.y=%d mLastPos.y=%d\n",
+                    this, mRestorePos.y, mLastPos.y);
 
   // Resolution properties should only exist on root scroll frames.
   MOZ_ASSERT(mIsRoot || aState->resolution() == 1.0);