I have a store asset, MaterialUI which I have been using. In 2017.2 I am having problems that don’t exist in 2017.1 or 5.6 and I am trying to track it down.
Essentially some of the included controls don’t layout properly when placed into a HorizontalLayoutGroup.
Has 2017.2 made some changes to how these layout groups work?
It seems you found a regression in 2017.2, thus I recommend to submit a bug-report following the advice in this document, to help that this issue does not occur in future Unity releases.
Using the bug-reporter seems to be an important step, because it makes sure the report is in Unity Technologies bug-tracking pipeline and has to be processed at some point. Using the forum is often used to add to a little more attention to a bug-report, but does not replace actually submitting a bug-report.
It’s from advantage to attach a project to the bug-report that UT can use to reproduce the issue and test their fix against. The easier an issue can be reproduced by QA, the more likely it is to get forwarded to a developer, who might or might not work on a bug-fix for that at some point.
After you submitted the bug-report, you receive a confirmation email with a Case number. UT often asks us to post the Case number in the forum thread, which allows them to find that bug-report if they look at your post.
Following these steps will increase the chance that UT is looking at your issue tremendously.
Yes I fully plan to submit a bug report (have submitted lots :p) but I wanted to narrow the scope more so that my report would be more useful, so I am in the research phase to help me do that 
If however there is no known or intentional change that is fine. I can do a bit more digging today and then submit a report with whatever I have found tomorrow.