LibWeb: Implement Selection.collapse_to_start

Userland/Libraries/LibWeb/Selection/Selection.cpp

@@ -199,9 +199,23 @@ WebIDL::ExceptionOr<void> Selection::set_position(JS::GCPtr<DOM::Node> node, uns
 }
 
 // https://w3c.github.io/selection-api/#dom-selection-collapsetostart
-void Selection::collapse_to_start()
+WebIDL::ExceptionOr<void> Selection::collapse_to_start()
 {
-    TODO();
+    // 1. The method must throw InvalidStateError exception if the this is empty.
+    if (!m_range) {
+        return WebIDL::InvalidStateError::create(realm(), "Selection.collapse_to_start() on empty range"sv);
+    }
+
+    // 2. Otherwise, it must create a new range
+    auto new_range = DOM::Range::create(*m_document);
+
+    // 3. Set the start both its start and end to the start of this's range
+    TRY(new_range->set_start(*anchor_node(), m_range->start_offset()));
+    TRY(new_range->set_end(*anchor_node(), m_range->start_offset()));
+
+    // 4. Then set this's range to the newly-created range.
+    m_range = new_range;
+    return {};
 }
 
 // https://w3c.github.io/selection-api/#dom-selection-collapsetoend

Userland/Libraries/LibWeb/Selection/Selection.h

@@ -39,7 +39,7 @@ public:
     void empty();
     WebIDL::ExceptionOr<void> collapse(JS::GCPtr<DOM::Node>, unsigned offset);
     WebIDL::ExceptionOr<void> set_position(JS::GCPtr<DOM::Node>, unsigned offset);
-    void collapse_to_start();
+    WebIDL::ExceptionOr<void> collapse_to_start();
     void collapse_to_end();
     WebIDL::ExceptionOr<void> extend(JS::NonnullGCPtr<DOM::Node>, unsigned offset);
     WebIDL::ExceptionOr<void> set_base_and_extent(JS::NonnullGCPtr<DOM::Node> anchor_node, unsigned anchor_offset, JS::NonnullGCPtr<DOM::Node> focus_node, unsigned focus_offset);