Thanks for the report and the video. This was indeed a regression of an earlier autocomplete sizing problem.
The change in Studio 26.8.6 that stopped long method signatures being truncated allowed the detail tooltip to wrap onto multiple lines. That changed the popup’s height while navigating suggestions and re-exposed a macOS rendering bug that could scale the popup contents incorrectly.
I've now fixed this more robustly: both the autocomplete list and its detail tooltip are rendered inside Studio’s overlay layer instead of separate resizable native macOS popup surfaces. Wrapped signatures remain fully visible, but the text size no longer changes.
The fix will be included in the next release: https://feedback.objo.dev/bug/1348