PixelPaint: Make move_selection() cycle through layers

Previously move_selection() did not work as expected. Instead store the
selected layer index in a member variable and continue to cycle through
the layers when you come to the start/end. Also use it to scroll into
view. Lastly rename the function to cycle_through_selection() to make it
clearer what it does.

Userland/Applications/PixelPaint/LayerListWidget.cpp

@@ -140,6 +140,7 @@ void LayerListWidget::mousedown_event(GUI::MouseEvent& event)
         return;
     }
     m_moving_gadget_index = gadget_index;
+    m_selected_layer_index = gadget_index.value();
     m_moving_event_origin = translated_event_point;
     auto& gadget = m_gadgets[m_moving_gadget_index.value()];
     auto& layer = m_image->layer(gadget_index.value());
@@ -190,6 +191,7 @@ void LayerListWidget::context_menu_event(GUI::ContextMenuEvent& event)
     auto gadget_index = gadget_at(translated_event_point);
     if (gadget_index.has_value()) {
         auto& layer = m_image->layer(gadget_index.value());
+        m_selected_layer_index = gadget_index.value();
         set_selected_layer(&layer);
     }
 
@@ -244,6 +246,7 @@ void LayerListWidget::select_bottom_layer()
 {
     if (!m_image || !m_image->layer_count())
         return;
+    m_selected_layer_index = 0;
     set_selected_layer(&m_image->layer(0));
 }
 
@@ -251,15 +254,25 @@ void LayerListWidget::select_top_layer()
 {
     if (!m_image || !m_image->layer_count())
         return;
+    m_selected_layer_index = m_image->layer_count() - 1;
     set_selected_layer(&m_image->layer(m_image->layer_count() - 1));
 }
 
-void LayerListWidget::move_selection(int delta)
+void LayerListWidget::cycle_through_selection(int delta)
 {
     if (!m_image || !m_image->layer_count())
         return;
-    int new_layer_index = min(max(0, (int)m_image->layer_count() + delta), (int)m_image->layer_count() - 1);
-    set_selected_layer(&m_image->layer(new_layer_index));
+
+    int selected_layer_index = static_cast<int>(m_selected_layer_index);
+    selected_layer_index += delta;
+
+    if (selected_layer_index < 0)
+        selected_layer_index = m_image->layer_count() - 1;
+    if (selected_layer_index > static_cast<int>(m_image->layer_count()) - 1)
+        selected_layer_index = 0;
+
+    m_selected_layer_index = selected_layer_index;
+    set_selected_layer(&m_image->layer(m_selected_layer_index));
 }
 
 void LayerListWidget::relayout_gadgets()
@@ -295,6 +308,8 @@ void LayerListWidget::set_selected_layer(Layer* layer)
         m_image->layer(i).set_selected(layer == &m_image->layer(i));
     if (on_layer_select)
         on_layer_select(layer);
+
+    scroll_into_view(m_gadgets[m_selected_layer_index].rect, false, true);
     update();
 }
 

Userland/Applications/PixelPaint/LayerListWidget.h

@@ -27,7 +27,7 @@ public:
 
     void select_bottom_layer();
     void select_top_layer();
-    void move_selection(int delta);
+    void cycle_through_selection(int delta);
 
 private:
     explicit LayerListWidget();
@@ -66,6 +66,8 @@ private:
 
     Optional<size_t> m_moving_gadget_index;
     Gfx::IntPoint m_moving_event_origin;
+
+    size_t m_selected_layer_index { 0 };
 };
 
 }

Userland/Applications/PixelPaint/main.cpp

@@ -350,12 +350,12 @@ int main(int argc, char** argv)
     layer_menu.add_separator();
     layer_menu.add_action(GUI::Action::create(
         "Select &Previous Layer", { 0, Key_PageUp }, [&](auto&) {
-            layer_list_widget.move_selection(1);
+            layer_list_widget.cycle_through_selection(1);
         },
         window));
     layer_menu.add_action(GUI::Action::create(
         "Select &Next Layer", { 0, Key_PageDown }, [&](auto&) {
-            layer_list_widget.move_selection(-1);
+            layer_list_widget.cycle_through_selection(-1);
         },
         window));
     layer_menu.add_action(GUI::Action::create(