Headless Avalonia ListBox+VirtualizingStackPanel spike answers all five
questions from the Phase 2a gate task: containers stay bounded at 1000
items, the existing InputHitTest ghost-drag model survives recycling,
variable row heights don't drift the Extent (only naive index*height
math would), auto-scroll via Offset mutation is mechanically feasible,
and mixed header/task rows template correctly via implicit DataTemplates.
Recommendation: proceed to Phase 2b. Spike code itself is throwaway and
not checked in.
mika kuns
2a8991496f