Add Sticky Header Offset

[hugoecken]

Problem Statement

When migrating from a SectionList to FlashList, I ran into a blocking issue with sticky section headers + an absolute app header.
In SectionList, I was able to add an offset so that sticky section headers would “stick” below my app’s absolute header (which is completely independent of the list, not a ListHeaderComponent).
In FlashList, sticky headers always stick at the very top of the viewport, behind my absolute application header.
There seems to be no way to offset them to respect the space taken by an app-level absolute header.
This prevents reproducing the same UX we had with SectionList.

Proposed Solution

Add a way to specify a sticky header top offset (for example, a prop like stickyHeaderTopOffset={number}).
This offset would define the Y position where section headers should start sticking, instead of always sticking at the very top.
Equivalent behavior was achievable with SectionList by combining contentInset and stickyHeaderIndices.
With such an API, developers could place a fixed header outside of the list and still have section headers stick just below it.

Priority

• Critical - Blocking my project
• High - Significant improvement to my workflow
• Medium - Would be nice to have
• Low - Minor enhancement

Checklist

• I've searched existing issues and discussions
• I've provided clear use cases for this feature
• I'm willing to help test this feature if implemented

Im using V2.1.0 with new arch.