LibWeb: Use correct boundary point comparison in Selection.extend

Previously Selection.extend() used only the relative node order to decide which
direction to extend the selection. This leads to incorrect behaviour if
both the existing and new boundary points are within the same DOM node
and the selection direction is reversed.

This change fixes all the failing subtests in the WPT extend-* test
suites.

Tests/LibWeb/Text/expected/selection-extend-across-siblings.txt

@@ -0,0 +1 @@
+PASS

Tests/LibWeb/Text/expected/selection-extend-backwards.txt

@@ -0,0 +1 @@
+PASS

Tests/LibWeb/Text/input/selection-extend-across-siblings.html

@@ -0,0 +1,22 @@
+<script src="include.js"></script>
+<p id="p1">Well hello</p>
+<p id="p2">friends</p>
+
+<script>
+    test(() => {
+        var selection = window.getSelection();
+        selection.setBaseAndExtent(p1.firstChild, 0, p1.firstChild, 0);
+        selection.extend(p2.firstChild, 4);
+        if (selection.anchorNode !== p1.firstChild
+            || selection.anchorOffset !== 0) {
+            println('FAIL: anchor has moved');
+            return;
+        }
+        if (selection.focusNode !== p2.firstChild
+            || selection.focusOffset !== 4) {
+            println('FAIL: focus is not where we expected');
+            return;
+        }
+        println('PASS');
+    })
+</script>

Tests/LibWeb/Text/input/selection-extend-backwards.html

@@ -0,0 +1,21 @@
+<script src="include.js"></script>
+<p id="a">Uno</p>
+
+<script>
+    test(() => {
+        var selection = window.getSelection();
+        selection.setBaseAndExtent(a.firstChild, 3, a.firstChild, 3);
+        selection.extend(a.firstChild, 0);
+        if (selection.anchorNode !== a.firstChild
+            || selection.anchorOffset !== 3) {
+            println('FAIL: anchor has moved');
+            return;
+        }
+        if (selection.focusNode !== a.firstChild
+            || selection.focusOffset !== 0) {
+            println('FAIL: focus is not where we expected');
+            return;
+        }
+        println('PASS');
+    })
+</script>

Userland/Libraries/LibWeb/Selection/Selection.cpp

@@ -290,7 +290,7 @@ WebIDL::ExceptionOr<void> Selection::extend(JS::NonnullGCPtr<DOM::Node> node, un
         TRY(new_range->set_end(new_focus_node, new_focus_offset));
     }
     // 6. Otherwise, if oldAnchor is before or equal to newFocus, set the start newRange's start to oldAnchor, then set its end to newFocus.
-    else if (old_anchor_node.is_before(new_focus_node) || &old_anchor_node == new_focus_node.ptr()) {
+    else if (position_of_boundary_point_relative_to_other_boundary_point(old_anchor_node, old_anchor_offset, new_focus_node, new_focus_offset) != DOM::RelativeBoundaryPointPosition::After) {
         TRY(new_range->set_start(old_anchor_node, old_anchor_offset));
         TRY(new_range->set_end(new_focus_node, new_focus_offset));
     }
@@ -304,7 +304,7 @@ WebIDL::ExceptionOr<void> Selection::extend(JS::NonnullGCPtr<DOM::Node> node, un
     set_range(new_range);
 
     // 9. If newFocus is before oldAnchor, set this's direction to backwards. Otherwise, set it to forwards.
-    if (new_focus_node->is_before(old_anchor_node)) {
+    if (position_of_boundary_point_relative_to_other_boundary_point(new_focus_node, new_focus_offset, old_anchor_node, old_anchor_offset) == DOM::RelativeBoundaryPointPosition::Before) {
         m_direction = Direction::Backwards;
     } else {
         m_direction = Direction::Forwards;