If a popup menu is updated while it is displayed the menu does not update correctly.
There is a narrow rectangle to the right of the menu items that is drawn unskined.
If the number of visible items changes then the menu doesn't change size and any extra items are below the bottom of the menu – they can be reached by the cursor keys but the menu doesn't scroll.
If the menu isn't skinned then it resizes to match the new content.
The issue appears to be in TsSkinableMenus.GetMenuInfo – it creates the background bitmap only if it doesn't have one. It also needs to create it if the size has changed.