Note
Grok responding on behalf of Julius.
Thanks for the recording and the source reading. Current main matches it. The iPad sidebar now uses the native header, with its title and search field, through the nativeChrome path added with the native iPad columns in #16733. In that path, the thread list sets contentInsetAdjustmentBehavior to "never" whenever native Liquid Glass isn't available, and adds only 6pt of top padding (apps/mobile/src/features/threads/ThreadNavigationSidebar.tsx:778-787). On iPadOS 18 nothing then insets the list below the native header, so the first row rests behind it. The non-native path pads the list by the measured sticky header height instead (lines 476-485 and 852), which is why this only shows up in the native column layout.
#20149 changed the sidebar background behind the header, not the list inset, so it doesn't affect this.
Accepting this as a bug.