LibWeb: Avoid infinite loop in HTMLElement.scrollParent

We were failing to actually climb up the containing block chain,
causing this API to infinite loop for anything but the most
trivial cases.

By fixing the loop structure, we also make a bunch of the already
imported WPT tests pass. :^)

Libraries/LibWeb/DOM/Document.h

@@ -98,6 +98,7 @@ enum class InvalidateLayoutTreeReason {
     X(HTMLElementOffsetParent)             \
     X(HTMLElementOffsetTop)                \
     X(HTMLElementOffsetWidth)              \
+    X(HTMLElementScrollParent)             \
     X(HTMLEventLoopRenderingUpdate)        \
     X(HTMLImageElementHeight)              \
     X(HTMLImageElementWidth)               \

Libraries/LibWeb/HTML/HTMLElement.cpp

@@ -464,6 +464,9 @@ String HTMLElement::outer_text()
 // https://drafts.csswg.org/cssom-view/#dom-htmlelement-scrollparent
 GC::Ptr<DOM::Element> HTMLElement::scroll_parent() const
 {
+    // NOTE: We have to ensure that the layout is up-to-date before querying the layout tree.
+    const_cast<DOM::Document&>(document()).update_layout(DOM::UpdateLayoutReason::HTMLElementScrollParent);
+
     // 1. If any of the following holds true, return null and terminate this algorithm:
     //    - The element does not have an associated box.
     //    - The element is the root element.
@@ -478,7 +481,7 @@ GC::Ptr<DOM::Element> HTMLElement::scroll_parent() const
 
     // 2. Let ancestor be the containing block of the element in the flat tree and repeat these substeps:
     auto ancestor = layout_node()->containing_block();
-    while (true) {
+    while (ancestor) {
         // 1. If ancestor is the initial containing block, return the scrollingElement for the element’s document if it
         //    is not closed-shadow-hidden from the element, otherwise return null.
         if (ancestor->is_viewport()) {
@@ -490,7 +493,8 @@ GC::Ptr<DOM::Element> HTMLElement::scroll_parent() const
 
         // 2. If ancestor is not closed-shadow-hidden from the element, and is a scroll container, terminate this
         //    algorithm and return ancestor.
-        if (!ancestor->dom_node()->is_closed_shadow_hidden_from(*this) && ancestor->is_scroll_container()) {
+        if ((ancestor->dom_node() && !ancestor->dom_node()->is_closed_shadow_hidden_from(*this))
+            && ancestor->is_scroll_container()) {
             return const_cast<Element*>(static_cast<DOM::Element const*>(ancestor->dom_node()));
         }
 
@@ -498,8 +502,10 @@ GC::Ptr<DOM::Element> HTMLElement::scroll_parent() const
         //    position containing block, terminate this algorithm and return null.
 
         // 4. Let ancestor be the containing block of ancestor in the flat tree.
-        ancestor = layout_node()->containing_block();
+        ancestor = ancestor->containing_block();
     }
+
+    return nullptr;
 }
 
 // https://www.w3.org/TR/cssom-view-1/#dom-htmlelement-offsetparent

Tests/LibWeb/Text/expected/HTML/scrollParent-infinite-loop.txt

@@ -0,0 +1 @@
+PASS (didn't hang)

Tests/LibWeb/Text/expected/wpt-import/css/cssom-view/scrollParent-shadow-tree.txt

@@ -2,8 +2,9 @@ Harness status: OK
 
 Found 4 tests
 
-4 Fail
+3 Pass
-Fail	scrollParent skips intermediate closed shadow tree nodes
+1 Fail
+Pass	scrollParent skips intermediate closed shadow tree nodes
 Fail	scrollParent skips intermediate open shadow tree nodes
-Fail	scrollParent from inside closed shadow tree
+Pass	scrollParent from inside closed shadow tree
-Fail	scrollParent from inside open shadow tree
+Pass	scrollParent from inside open shadow tree

Tests/LibWeb/Text/expected/wpt-import/css/cssom-view/scrollParent.txt

@@ -2,14 +2,14 @@ Harness status: OK
 
 Found 9 tests
 
-3 Pass
+7 Pass
-6 Fail
+2 Fail
-Fail	scrollParent returns the nearest scroll container.
+Pass	scrollParent returns the nearest scroll container.
-Fail	hidden element is a scroll container.
+Pass	hidden element is a scroll container.
 Pass	Element with no box has null scrollParent.
-Fail	scrollParent follows absolute positioned containing block chain.
+Pass	scrollParent follows absolute positioned containing block chain.
 Fail	scrollParent follows fixed positioned containing block chain.
-Pass	scrollParent of element fixed to root is null.
+Fail	scrollParent of element fixed to root is null.
-Fail	scrollParent of child in root viewport returns document scrolling element.
+Pass	scrollParent of child in root viewport returns document scrolling element.
-Fail	scrollParent of fixed element contained within root is document scrolling element.
+Pass	scrollParent of fixed element contained within root is document scrolling element.
 Pass	scrollParent of body is null.

Tests/LibWeb/Text/input/HTML/scrollParent-infinite-loop.html

@@ -0,0 +1,9 @@
+<!doctype html>
+<script src="../include.js"></script>
+<body><div id=foo><div id=bar></div></div>
+<script>
+    test(() => {
+        bar.scrollParent;
+        println("PASS (didn't hang)");
+    });
+</script>