With configuration variables from change I47db5eab4569514d039261d11b6dedb0eeae17b5 it's possible to force the use of MediaWikiUI whereever it's supported. In the future it will (maybe?) be standard to use mediawikiui everwhere. Actual, the padding css rule of WikiEditor results in an ugly overlapping with the blue bar of mw ui (indicating, that this field is focused).
I'm not sure, if there are special reasons for the padding, but i would suggest to just remove the rule or don't set it for padding-left.