I found this thread because I just ran into a similar issue. I have a “user” horizontal menu at the end of the menu bar, and when I click the user for a dropdown, everything to the left of it shifts underneath it. Wrapping it in another container does indeed fix it, but when I set the container’s basis to auto I get the same problem.
This is a result of how we render the Horizontal Menu in some situations. The internal ticket clarifies that we are aware of this behavior when the horizontal menu is in a Docked View - is that the situation you are encountering?
We do have an open ticket to repair this behavior, but until it is repaired you’ll need to have a defined basis for the wrapping container of the Horizontal Menu; a basis value of auto results in the wrapping container displaying the same behavior other users had gotten around by using the container in the first place because that setting results in the container taking up no space because the horizontal menu is taking up no space…