Add onShowFinished callback to BoxContent.

ui/layers/box_content.h

@@ -126,6 +126,8 @@ public:
 
 	void scrollToWidget(not_null<QWidget*> widget);
 
+	virtual void showFinished() {
+	}
 	void clearButtons() {
 		getDelegate()->clearButtons();
 	}

ui/layers/box_layer_widget.h

@@ -53,6 +53,10 @@ public:
 		LayerOptions options,
 		anim::type animated) override;
 
+	void showFinished() override {
+		_content->showFinished();
+	}
+
 	void clearButtons() override;
 	QPointer<RoundButton> addButton(
 		rpl::producer<QString> text,

ui/layers/generic_box.h

@@ -38,6 +38,9 @@ public:
 	void setFocusCallback(Fn<void()> callback) {
 		_focus = callback;
 	}
+	void setShowFinishedCallback(Fn<void()> callback) {
+		_showFinished = callback;
+	}
 
 	int rowsCount() const {
 		return _content->count();
@@ -76,6 +79,11 @@ public:
 			BoxContent::setInnerFocus();
 		}
 	}
+	void showFinished() override {
+		if (_showFinished) {
+			_showFinished();
+		}
+	}
 
 	[[nodiscard]] not_null<Ui::VerticalLayout*> verticalLayout();
 
@@ -111,6 +119,7 @@ private:
 
 	FnMut<void(not_null<GenericBox*>)> _init;
 	Fn<void()> _focus;
+	Fn<void()> _showFinished;
 	object_ptr<Ui::VerticalLayout> _content;
 	int _width = 0;