The header hides while the gallery's content is scrolled down and returns at its top. Switching
tabs hides the old tab's pane (or removes its map) without any scroll event, so after scrolling a
tab down and switching to a map-only tab (Prayer Progress, Cities with Internet, Gospel Resources
on a phone) the header stayed hidden, and nothing on that tab could bring it back. The page now
remembers which scroller hid the header; when a change in the gallery hides or removes it, the
header returns. The new tab starts with the header shown, and if it scrolls, the next scroll hides
it again.