@Krinkle thanks for documenting this. Regarding it moving up: we're actually working on fixing that here T279015 (see T279015#7170083), as a follow-up to a related bug. Apologies for the task description now being out of sync with the work being done (I thought it might cause more confusion if I changed it). @Jdlrobson do you want to submit your new patches on this task, or merge this into T279015?

@Krinkle regarding the search icon moving to the left, and your comment/question in the description:

The box also widens to the left, presumably for article thumbnails to fit later on, but it's not clear why the wider box with search icon is "good" design on focus, but "bad" for the same situation off-focus. Maybe it's good for both scenarios. Or maybe the box doesn't need to widen if the title is allowed to not be directly under the input (like on https://www.wikipedia.org/, and Minerva, and OOUI/VisualEditor already).

The reason why it widens is indeed so there is room for the thumbnail while ensuring that the text in the results list lines up horizontally with the text in the search field (which we believe makes for a smoother transition for your eye as you look down from your search term to the search results) . You can see the discussion here: https://phabricator.wikimedia.org/T255603#6294135. Regarding whether it could always have that spacing (non-focus & focus) that was @Volker_E's call, based on our style guide.

We are however hoping to make the shift less noticeable by waiting for the search results menu to appear: