The 78s sweep fired regardless of whether anyone was looking, spending mixnet
round-trips in the background. Gate it on a frame heartbeat: the GUI stamps
crate::mark_frame() each draw (with a light ~2s repaint cadence so it stays
fresh while visible); when the app is backgrounded eframe stops drawing and the
stamp goes stale, so crate::app_foreground() reads false and the sweep skips.
The skip deliberately does NOT advance last_name_sweep, so the first tick after
the app returns to the foreground runs the sweep immediately — catching up on
resume rather than waiting out another full interval. Heartbeat lives at the
crate root so nostr reads it without depending on the gui module.
2ro
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