| buttonRect | Is used for positioning the window. |
| windowSize | Is used for setting up initial size of the window. |
Show window with dropdown behaviour (e.g. window is closed when it loses focus) and having.
Appearance as 'ShowMode.PopupMenu' (no frame). Note this function auto fits the window to screen while trying to place it first below then above. So windowSize might change when fitting it to screen therefore make sure to get 'position' afterwards to check size was cropped.