This issue follows on from MDL-75978. Before that issue, there were three problems found (with the second and third following from the first.

To reproduce:

1. Import the attached problem-ddwtos-question.xml (it is Moodle XML format for import into the question bank.)
2. Create a quiz, and add this question.
3. On an iPhone, log in as a student and attempt the quiz. (Not sure if any other device/brower combinations are affected.)
4. Try answering this question, and note the weird behaviour. (Described just below, and see attached videos.)

The problems observed are:

1. Sometimes, as you drag, the last drag items on one line randomly jump down to to the start of the next line and back. Problem_1_video.mp4
2. This random movement caused the code to put one drag item in two different drop zones. This left the question in a broken state, after which it was not possible to drag items any more at all. Instead you got errors in the JS console. <--- This is what MDL-75978 fixed, so should no longer be an issue.
3. Another effect of the random jiggling is that sometimes a different drag item would be sent back home when dropping on a particular drop position. Problem_3_video.mp4

This issue exists to fix points 1. and 3. which are not fixed yet.

In MDL-75978 some CSS was found which made the problem go away, but only by using CSS or HTML changes which are likely to break the layout of some questions, with certain content in the question text, so we need a better fix.

Really, I think the random jiggling is a Safari bug, but we can't do anything about that, so we probably need to find a work-around to make the layout robust.