When a toolbar with several items is resized so that not all items can be displayed at once anymore, some items should be "moved" to the overflow container. I am, of course, assuming that these items have "overflow" set to "auto", which is the default value. As soon as at least one item is overflown, the overflow button should be displayed. This DOJO creates such an environment.
Now, when you resize the page so that the overflow is activated, you can see that not enough space is reserved for the overflow button. It is (first partially, then fully) hidden behind the rightmost item before this item is finally "moved" to the overflow container.
If you look closely, you can still see parts of the three dots behind the "Toggle" item.