[css-scroll-snap] only preserve a target if we have targets for both …

…axes

https://bugs.webkit.org/show_bug.cgi?id=245989
<rdar://100737724>

Reviewed by Simon Fraser.

The preserving of a particular snap target was intended to follow this line
of the spec: "If multiple boxes were snapped before and their snap positions
no longer coincide, then if one of them is focused or targeted, the scroll
container must re-snap to that one and otherwise which one to re-snap to is
UA-defined". Since it is specified that this is for the case of mulitple boxes
being snapped, change the logic so that we only preserve a target if we are
snapped to multiple boxes.

* LayoutTests/TestExpectations:
* Source/WebCore/platform/ScrollSnapAnimatorState.cpp:
(WebCore::ScrollSnapAnimatorState::setFocusedElementForAxis):
(WebCore::ScrollSnapAnimatorState::preserveCurrentTargetForAxis):
(WebCore::ScrollSnapAnimatorState::resnapAfterLayout):

Canonical link: https://commits.webkit.org/255493@main

LayoutTests/TestExpectations

@@ -6066,8 +6066,6 @@ imported/w3c/web-platform-tests/html/semantics/scripting-1/the-script-element/cs
 imported/w3c/web-platform-tests/html/semantics/scripting-1/the-script-element/css-module/cors-crossorigin-requests.html [ Pass Failure ]
 imported/w3c/web-platform-tests/html/semantics/scripting-1/the-script-element/json-module/cors-crossorigin-requests.html [ Pass Failure ]
 
-webkit.org/b/245164 css3/scroll-snap/scroll-snap-drag-scrollbar-thumb-with-relayouts.html [ Failure Pass ]
-
 # Disable ShadowRealm (while running to ensure we do not crash)
 webkit.org/b/245680 http/tests/misc/iframe-shadow-realm.html [ Skip ]
 inspector/shadow-realm-console.html [ Pass Failure ]

Source/WebCore/platform/ScrollSnapAnimatorState.cpp

@@ -103,6 +103,7 @@ void ScrollSnapAnimatorState::setFocusedElementForAxis(ScrollEventAxis axis)
     });
     if (found == snapOffsets.end())
         return;
+
     auto newIndex = std::distance(snapOffsets.begin(), found);
     auto newID = snapOffsets[newIndex].snapTargetID;
     setActiveSnapIndexForAxis(axis, newIndex);
@@ -117,8 +118,10 @@ bool ScrollSnapAnimatorState::preserveCurrentTargetForAxis(ScrollEventAxis axis)
     auto found = std::find_if(snapOffsets.begin(), snapOffsets.end(), [snapID](SnapOffset<LayoutUnit> p) -> bool {
         return p.snapTargetID == *snapID;
     });
-    if (found == snapOffsets.end())
+    if (found == snapOffsets.end()) {
+        setActiveSnapIndexIDForAxis(axis, std::nullopt);
         return false;
+    }
 
     setActiveSnapIndexForAxis(axis, std::distance(snapOffsets.begin(), found));
     return true;
@@ -148,12 +151,11 @@ bool ScrollSnapAnimatorState::resnapAfterLayout(ScrollOffset scrollOffset, const
         activeHorizontalIndex = activeSnapIndexForAxis(ScrollEventAxis::Horizontal);
     }
 
-    // If we have an active target, see if we need to preserve it
-    if (activeHorizontalID && activeHorizontalIndex) {
-        if (*activeHorizontalID != snapOffsetsHorizontal[*activeHorizontalIndex].snapTargetID)
-            snapPointChanged |= preserveCurrentTargetForAxis(ScrollEventAxis::Horizontal);
-    } else if (activeVerticalID && activeVerticalIndex && *activeVerticalID != snapOffsetsVertical[*activeVerticalIndex].snapTargetID)
-        snapPointChanged |= preserveCurrentTargetForAxis(ScrollEventAxis::Vertical);
+    // If we have active targets, see if we need to preserve one of them
+    if (activeHorizontalID && activeHorizontalIndex && activeVerticalID && activeVerticalIndex && *activeHorizontalID
+        != snapOffsetsHorizontal[*activeHorizontalIndex].snapTargetID && *activeVerticalID
+        != snapOffsetsVertical[*activeVerticalIndex].snapTargetID)
+        snapPointChanged |= preserveCurrentTargetForAxis(ScrollEventAxis::Horizontal);
 
     // If we do not have current targets and are snapped to multiple targets, set them
     if ((!activeHorizontalID && activeHorizontalIndex) && (!activeVerticalID && activeVerticalIndex)) {