diff --git a/lib/cyberarm_engine/ui/element.rb b/lib/cyberarm_engine/ui/element.rb
index 3fda6a1..a09ea2d 100644
--- a/lib/cyberarm_engine/ui/element.rb
+++ b/lib/cyberarm_engine/ui/element.rb
@@ -18,6 +18,8 @@ module CyberarmEngine
       @visible = @options[:visible].nil? ? true  : @options[:visible]
       @tip     = @options[:tip] || ""
 
+      @debug_color = @options[:debug_color].nil? ? Gosu::Color::RED : @options[:debug_color]
+
       @style = Style.new(options)
 
       @root ||= nil
@@ -160,6 +162,31 @@ module CyberarmEngine
       end
     end
 
+    def debug_draw
+      return if defined?(GUI_DEBUG_ONLY_ELEMENT) && self.class == GUI_DEBUG_ONLY_ELEMENT
+
+      Gosu.draw_line(
+        x, y, @debug_color,
+        x + outer_width, y, @debug_color,
+        Float::INFINITY
+      )
+      Gosu.draw_line(
+        x + outer_width, y, @debug_color,
+        x + outer_width, y + outer_height, @debug_color,
+        Float::INFINITY
+      )
+      Gosu.draw_line(
+        x + outer_width, y + outer_height, @debug_color,
+        x, y + outer_height, @debug_color,
+        Float::INFINITY
+      )
+      Gosu.draw_line(
+        x, outer_height, @debug_color,
+        x, y, @debug_color,
+        Float::INFINITY
+      )
+    end
+
     def update
     end
 
diff --git a/lib/cyberarm_engine/ui/elements/container.rb b/lib/cyberarm_engine/ui/elements/container.rb
index 197796c..7a2dd6c 100644
--- a/lib/cyberarm_engine/ui/elements/container.rb
+++ b/lib/cyberarm_engine/ui/elements/container.rb
@@ -60,30 +60,13 @@ module CyberarmEngine
         Gosu.clip_to(@x, @y, width, height) do
           @children.each(&:draw)
         end
+      end
 
-        if false # DEBUG
-          Gosu.flush
+      def debug_draw
+        super
 
-          Gosu.draw_line(
-            x, y, Gosu::Color::RED,
-            x + outer_width, y, Gosu::Color::RED,
-            Float::INFINITY
-          )
-          Gosu.draw_line(
-            x + outer_width, y, Gosu::Color::RED,
-            x + outer_width, y + outer_height, Gosu::Color::RED,
-            Float::INFINITY
-          )
-          Gosu.draw_line(
-            x + outer_width, y + outer_height, Gosu::Color::RED,
-            x, y + outer_height, Gosu::Color::RED,
-            Float::INFINITY
-          )
-          Gosu.draw_line(
-            x, outer_height, Gosu::Color::RED,
-            x, y, Gosu::Color::RED,
-            Float::INFINITY
-          )
+        @children.each do |child|
+          child.debug_draw
         end
       end
 
@@ -206,8 +189,6 @@ module CyberarmEngine
       def mouse_wheel_up(sender, x, y)
         return unless @style.scroll
 
-        return unless height < scroll_height
-
         if @scroll_position.y < 0
           @scroll_position.y += @scroll_speed
           @scroll_position.y = 0 if @scroll_position.y > 0
@@ -238,7 +219,12 @@ module CyberarmEngine
       def scroll_top=(n)
         n = 0 if n <= 0
         @scroll_position.y = -n
-        @scroll_position.y = -max_scroll_height if @scroll_position.y.abs > max_scroll_height
+
+        if max_scroll_height.positive?
+          @scroll_position.y = -max_scroll_height if @scroll_position.y.abs > max_scroll_height
+        else
+          @scroll_position.y = 0
+        end
       end
 
       def value
diff --git a/lib/cyberarm_engine/ui/gui_state.rb b/lib/cyberarm_engine/ui/gui_state.rb
index 9e5bfa6..53004ef 100644
--- a/lib/cyberarm_engine/ui/gui_state.rb
+++ b/lib/cyberarm_engine/ui/gui_state.rb
@@ -55,6 +55,12 @@ module CyberarmEngine
         Gosu.flush
         @tip.draw
       end
+
+      if defined?(GUI_DEBUG)
+        Gosu.flush
+
+        @root_container.debug_draw
+      end
     end
 
     def update
diff --git a/lib/cyberarm_engine/ui/theme.rb b/lib/cyberarm_engine/ui/theme.rb
index 5f218b1..e6c6746 100644
--- a/lib/cyberarm_engine/ui/theme.rb
+++ b/lib/cyberarm_engine/ui/theme.rb
@@ -64,6 +64,10 @@ module CyberarmEngine
         border_radius: 0
       },
 
+      Container: { # < Element (Base class for Stack and Flow)
+        debug_color: Gosu::Color::YELLOW
+      },
+
       Button: { # < Label
         margin: 1,
         padding: 4,