Bug 1615955 - Suppress scroll anchoring when scroll offset is zero. r=dholbert

As per https://github.com/w3c/csswg-drafts/pull/4679.

Differential Revision: https://phabricator.services.mozilla.com/D63761

--HG--
extra : moz-landing-system : lando

layout/generic/ScrollAnchorContainer.cpp

@@ -382,16 +382,19 @@ void ScrollAnchorContainer::ApplyAdjustments() {
       mScrollFrame->HasPendingScrollRestoration() ||
       mScrollFrame->IsProcessingScrollEvent() ||
       mScrollFrame->IsProcessingAsyncScroll() ||
-      mScrollFrame->mApzSmoothScrollDestination.isSome()) {
+      mScrollFrame->mApzSmoothScrollDestination.isSome() ||
+      mScrollFrame->GetScrollPosition() == nsPoint()) {
     ANCHOR_LOG(
         "Ignoring post-reflow (anchor=%p, dirty=%d, disabled=%d, "
         "pendingRestoration=%d, scrollevent=%d, asyncScroll=%d, "
-        "apzSmoothDestination=%d, pendingSuppression=%d, container=%p).\n",
+        "apzSmoothDestination=%d, zeroScrollPos=%d pendingSuppression=%d, "
+        "container=%p).\n",
         mAnchorNode, mAnchorNodeIsDirty, mDisabled,
         mScrollFrame->HasPendingScrollRestoration(),
         mScrollFrame->IsProcessingScrollEvent(),
         mScrollFrame->IsProcessingAsyncScroll(),
         mScrollFrame->mApzSmoothScrollDestination.isSome(),
+        mScrollFrame->GetScrollPosition() == nsPoint(),
         mSuppressAnchorAdjustment, this);
     if (mSuppressAnchorAdjustment) {
       mSuppressAnchorAdjustment = false;

testing/web-platform/meta/css/css-scroll-anchoring/zero-scroll-offset.html.ini

@@ -1,16 +0,0 @@
-[zero-scroll-offset.html]
-  [Scroll anchoring suppressed when scroll offset is zero.]
-    expected:
-      if (os == "win") and debug and not webrender and not fission and (processor == "x86"): ["PASS", "FAIL"]
-      if (os == "linux") and not fission and (processor == "x86_64") and debug and not webrender: ["PASS", "FAIL"]
-      if (os == "linux") and not fission and (processor == "x86_64") and debug and webrender: ["PASS", "FAIL"]
-      if (os == "linux") and not fission and (processor == "x86_64") and not debug and webrender: ["PASS", "FAIL"]
-      if (os == "linux") and not fission and (processor == "x86_64") and not debug and not webrender: ["PASS", "FAIL"]
-      if (os == "win") and debug and webrender and fission: ["PASS", "FAIL"]
-      if (os == "win") and not debug and not fission and webrender: FAIL
-      if (os == "linux") and fission and not debug and not webrender: ["PASS", "FAIL"]
-      if (os == "linux") and fission and debug and webrender: ["PASS", "FAIL"]
-      if (os == "linux") and fission and debug and not webrender: ["PASS", "FAIL"]
-      if (os == "win") and not debug and fission: FAIL
-      [FAIL, PASS]
-