e-paper RAM persists between refreshes, but the decoder reset the accumulated
window union on every 0x20 MASTER_ACTIVATION. A PARTIAL refresh (BadgeOS redraws
only a strip when you navigate) then set a small window, so composeFrame emitted
just that strip, placed top-left with the rest black — reading as a 90°-rotated
screen with a black band down the side. Keep the union (and the greyscale flag)
across refreshes so it stays at the full-panel extent set by the first full
refresh; every frame now composes the whole persistent RAM and partial updates
land in place. Cleared only by a real RST (0x12). Paged full refreshes (GxEPD2)
re-cover the panel each frame so they are unchanged; the BadgeOS menu render is
byte-identical and all 23 decoder/epaper tests pass.
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