PixelPaint: Implement automatic scrolling in LayerListWidget

The previous implementation of automatic scrolling in LayerListWidget
relied on mousemove events to perform the scrolling, which didn't
produce the expected behavior. Instead use the new auto scroll timer.

Userland/Applications/PixelPaint/LayerListWidget.cpp

@@ -175,15 +175,24 @@ void LayerListWidget::mousemove_event(GUI::MouseEvent& event)
         return;
 
     Gfx::IntPoint translated_event_point = { 0, vertical_scrollbar().value() + event.y() };
-
     auto delta = translated_event_point - m_moving_event_origin;
+
+    if (delta.y() == 0)
+        return;
+
     auto& gadget = m_gadgets[m_moving_gadget_index.value()];
     VERIFY(gadget.is_moving);
-    gadget.movement_delta = delta;
 
-    auto adjusted_rect = gadget.rect;
+    gadget.movement_delta.set_y(delta.y());
-    adjusted_rect.translate_by(gadget.movement_delta);
+    auto inner_rect_max_height = widget_inner_rect().height() - 2 + vertical_scrollbar().max();
-    scroll_into_view(adjusted_rect, false, true);
+
+    if (delta.y() < 0 && gadget.rect.y() < -delta.y())
+        gadget.movement_delta.set_y(-gadget.rect.y());
+    else if (delta.y() > 0 && gadget.rect.bottom() + delta.y() > inner_rect_max_height)
+        gadget.movement_delta.set_y(inner_rect_max_height - gadget.rect.bottom());
+
+    m_automatic_scroll_delta = automatic_scroll_delta_from_position(event.position());
+    set_automatic_scrolling_timer(vertical_scrollbar().is_scrollable() && !m_automatic_scroll_delta.is_null());
 
     relayout_gadgets();
 }
@@ -203,6 +212,7 @@ void LayerListWidget::mouseup_event(GUI::MouseEvent& event)
         new_index = m_image->layer_count() - 1;
 
     m_moving_gadget_index = {};
+    set_automatic_scrolling_timer(false);
 
     auto old_layer_index = to_layer_index(old_index);
     auto new_layer_index = to_layer_index(new_index);
@@ -224,6 +234,34 @@ void LayerListWidget::context_menu_event(GUI::ContextMenuEvent& event)
         on_context_menu_request(event);
 }
 
+void LayerListWidget::on_automatic_scrolling_timer_fired()
+{
+    auto& gadget = m_gadgets[m_moving_gadget_index.value()];
+    VERIFY(gadget.is_moving);
+
+    if (m_automatic_scroll_delta.y() == 0)
+        return;
+
+    if (vertical_scrollbar().is_min() && m_automatic_scroll_delta.y() < 0)
+        return;
+
+    if (vertical_scrollbar().is_max() && m_automatic_scroll_delta.y() > 0)
+        return;
+
+    vertical_scrollbar().set_value(vertical_scrollbar().value() + m_automatic_scroll_delta.y());
+    gadget.movement_delta.set_y(gadget.movement_delta.y() + m_automatic_scroll_delta.y());
+
+    auto inner_rect_max_height = widget_inner_rect().height() - 2 + vertical_scrollbar().max();
+    auto gadget_absolute_position = gadget.rect.y() + gadget.movement_delta.y();
+
+    if (gadget_absolute_position < 0)
+        gadget.movement_delta.set_y(-gadget.rect.y());
+    else if (gadget_absolute_position + gadget.rect.height() >= inner_rect_max_height)
+        gadget.movement_delta.set_y(inner_rect_max_height - gadget.rect.bottom());
+    else
+        relayout_gadgets();
+}
+
 void LayerListWidget::image_did_add_layer(size_t layer_index)
 {
     if (m_moving_gadget_index.has_value()) {

Userland/Applications/PixelPaint/LayerListWidget.h

@@ -45,6 +45,7 @@ private:
     virtual void image_did_modify_layer_properties(size_t) override;
     virtual void image_did_modify_layer_bitmap(size_t) override;
     virtual void image_did_modify_layer_stack() override;
+    virtual void on_automatic_scrolling_timer_fired() override;
 
     void rebuild_gadgets();
     void relayout_gadgets();
@@ -72,6 +73,8 @@ private:
     Optional<size_t> m_moving_gadget_index;
     Gfx::IntPoint m_moving_event_origin;
 
+    Gfx::IntPoint m_automatic_scroll_delta;
+
     size_t m_selected_gadget_index { 0 };
 };