[scroll-anchoring] Contract owning scroller rect for scroll padding

https://bugs.webkit.org/show_bug.cgi?id=262342
rdar://116209834

Reviewed by Simon Fraser.

Contract owning scroller rect for scroll padding.

* LayoutTests/imported/w3c/web-platform-tests/css/css-scroll-anchoring/scroll-padding-affects-anchoring-expected.txt:
* Source/WebCore/page/scrolling/ScrollAnchoringController.cpp:
(WebCore::ScrollAnchoringController::examineCandidate):

Canonical link: https://commits.webkit.org/270483@main

LayoutTests/imported/w3c/web-platform-tests/css/css-scroll-anchoring/scroll-padding-affects-anchoring-expected.txt

@@ -1,3 +1,3 @@
 
-FAIL scroll anchoring accounts for scroll-padding assert_equals: Shouldn't anchor to #changer, since it's covered by scroll-padding expected 150 but got 50
+PASS scroll anchoring accounts for scroll-padding
 

Source/WebCore/page/scrolling/ScrollAnchoringController.cpp

@@ -169,6 +169,15 @@ CandidateExaminationResult ScrollAnchoringController::examineCandidate(Element&
     auto containingRect = boundingRectForScrollableArea(m_owningScrollableArea);
     auto* document = frameView().frame().document();
 
+    if (auto* element = elementForScrollableArea(m_owningScrollableArea)) {
+        if (auto* box = element->renderBox()) {
+            LayoutRect paddedLayerBounds(containingRect);
+            paddedLayerBounds.contract(box->scrollPaddingForViewportRect(paddedLayerBounds));
+            LOG_WITH_STREAM(ScrollAnchoring, stream << "ScrollAnchoringController::examineCandidate() contracted rect: "<< IntRect(paddedLayerBounds));
+            containingRect = IntRect(paddedLayerBounds);
+        }
+    }
+
     if (auto renderer = element.renderer()) {
         // TODO: we need to think about position: absolute
         // TODO: figure out how to get scrollable area for renderer to check if it is maintaining scroll anchor