Change 584626 had a related patch set uploaded (by Awight; owner: Awight):
[mediawiki/extensions/TwoColConflict@master] [WIP] Browser tests for draggable talk page blocks

https://gerrit.wikimedia.org/r/584626

Did you try updating to webdriverio v6 (T247254)? There's some drag/drop improvements.

Announcement: https://webdriver.io/blog/2020/03/26/webdriverio-v6-released.html

Updating to v6 should be way easier than to v5, at least that's what the announcement says. 😁

In T249208#6023484, @zeljkofilipin wrote:

Did you try updating to webdriverio v6 (T247254)? There's some drag/drop improvements.

Thanks, this was totally worth trying. I upgraded and nothing changed, unfortunately.

My guess is that the way webdriver's drag-and-drop works is incompatible with oojs-ui, maybe because elements are detached from the DOM and reattached, or perhaps the buttonDown grabs the wrong element...