Steps to reproduce:

On a slow connection, type "Katze" in the upper right search bar of the German Wikipedia and select the "enthält ..." option at the bottom of the autocomplete drop-down.

Expected Result:
I see the results, with a white placeholder where the Advanced Search will appear. When the Advanced Search has finished loading, the white placeholder is replaced with the Advanced Search.

Actual result:
The search results are displayed directly below the default plain search field. When the Advanced Search has finished loading, the search results are pushed down and I lose my context or misclick.

Note:
This happens because we're "abusing" the old namespace selection (element #mw-searchoptions) as a hidden placeholder & setting its height (see T189028: Reduce or remove content jumps because of JavaScript loading times). This element is not present for the "simple" search result that is displayed when coming from the search bar.

There at least two solutions: Continue with Moriels work of prerendering on the server or insert our own placeholder and "container" until the whole element is loaded (probably the placeholder needs to be rendered on the server to avoid jumping). The container element should be slightly bigger (min-height) than the advanced search because it has slightly different heights on different platforms, which currently also leads to some slight jumpiness when not using the simple search, despite the efforts in T189028: Reduce or remove content jumps because of JavaScript loading times.

If we'd go with Moriel's solution, it would have been estimated as 13+.