The flag was set in _ensure_post_categories_async which runs AFTER
_on_post_selected calls info_panel.set_post. By the time the flag
was True, the flat tags had already rendered. The flash persisted.
Fix: check whether a fetch is needed and set the flag in
_on_post_selected, right before set_post. The info panel sees the
flag and skips the flat-tag fallback on its first render.