UI: Give views a pointer to their parent group

Set by ViewGroup when a view is added, inserted or swapped in. Lets a view
find the scroll view it's in, for example.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Common/UI/View.h

@@ -471,6 +471,9 @@ public:
 	const std::string &Tag() const { return tag_; }
 	void SetTag(std::string_view str) { tag_ = str; }
 
+	// The group this view was added to, if any.
+	ViewGroup *Parent() const { return parent_; }
+
 	// Fake RTTI
 	virtual bool IsViewGroup() const { return false; }
 	virtual bool ContainsSubview(const View *view) const { return false; }
@@ -518,6 +521,9 @@ protected:
 	DialogResult autoResult_ = DR_OK;
 
 private:
+	friend class ViewGroup;  // Sets parent_.
+	ViewGroup *parent_ = nullptr;
+
 	std::function<bool()> enabledFunc_;
 	bool *enabledPtr_ = nullptr;
 	bool enabled_ = true;

Common/UI/ViewGroup.cpp

@@ -164,6 +164,7 @@ bool ViewGroup::ReplaceSubview(View *view, View *newView) {
 	for (int i = 0; i < (int)views_.size(); i++) {
 		if (views_[i] == view) {
 			views_[i] = newView;
+			SetParentOf(newView);
 			delete view;
 			return true;
 		}
@@ -1042,8 +1043,7 @@ CollapsibleSection::CollapsibleSection(std::string_view title, LayoutParams *lay
 	open_ = &localOpen_;
 	SetSpacing(0.0f);
 
-	header_ = new CollapsibleHeader(open_, title);
-	views_.push_back(header_);
+	header_ = Add(new CollapsibleHeader(open_, title));
 	header_->OnClick.Add([=](UI::EventParams &) {
 		// Change the visibility of all children except the first one.
 		// Later maybe try something more ambitious.

Common/UI/ViewGroup.h

@@ -53,11 +53,13 @@ public:
 	template <class T>
 	T *Add(T *view) {
 		views_.push_back(view);
+		SetParentOf(view);
 		return view;
 	}
 	template <class T>
 	T *Insert(int index, T *view) {
 		views_.insert(views_.begin() + index, view);
+		SetParentOf(view);
 		return view;
 	}
 
@@ -108,6 +110,9 @@ protected:
 	std::string DescribeListUnordered(std::string_view heading) const;
 	std::string DescribeListOrdered(std::string_view heading) const;
 
+	// Anything that adds to views_ without Add or Insert must call this.
+	void SetParentOf(View *view) { view->parent_ = this; }
+
 	std::vector<View *> views_;
 	View *defaultFocusView_ = nullptr;
 	Drawable bg_;