Jemt
iOS < 10: DropDown with BoundaryDetection incorrectly positions picker when keyboard opens
iOS 9 and below does not work well with BoundaryDetection(true). The DropDown picker is incorrectly positioned if the user clicks the input area of the dropdown, in which case an input gains focus and the keyboard opens. The keyboard most likely changes the viewport while the control tries to calculate the optimum position for the picker. Apple seems to have realized the problem and made sure their browser report the expected viewport dimensions from iOS version 10 and up.
Today I tested with iOS 9.3.5 on an iPad Mini 1 - DropDown was initially positioned just fine with boundary detection enabled. However, selecting an item resulted in the keyboard being hidden, and this caused the pull down menu to become "detached" from the control, because the pull down menu has position:fixed, and the control "follows" the keyboard down to the bottom of the page. It could be that different minor versions of iOS 9 (and obviously major versions as well) behave differently. The screen size may also affect the behaviour.
It would help a lot (also on most recent versions of iOS) if the input field was not focused at all on mobile, unless InputEnabled is True. It's annoying to have the keyboard pop up when there is nothing we can use it for, and it does cause the page to move when it adjusts for the keyboard. Basically, on mobile with InputEnabled set to False, clicking anywhere on a dropdown should make it behave as if the arrow was clicked. The control gains focus and opens the pulldown menu - that's it.