ESP32 ePaper examples (e.g. epaper-2in9-esp32-weather) rendered as a blank
white panel. Two bugs, both above the SPI layer:
1. The worker's epaper_update event nested its payload under 'data', unlike
every other (flat) worker event. The backend qemu_callback re-wraps the
post-'type' payload under 'data', so the frontend received
msg.data.data.component_id (undefined) and EPaperPart bailed on
id !== componentId, so paintFrame/putImageData never ran. Emit it flat.
2. Ssd168xEpaperSlave was sized to the display dims (296x128), but GxEPD2
with setRotation(1) writes the controller's NATIVE RAM (128x296). The
_y < height bound dropped rows 128-295 (half the image) and compose
never rotated. Size RAM to the longer side, compose in the native
active-window geometry (0x44/0x45), then rotate to the display
orientation (inverse of Adafruit_GFX rotation 1). Add is_bwr (from
panel_kind): B/W panels init the 0x26 plane white and compose
white-only-if-both (GDEY029T94 mirrors the image into 0x26); tri-colour
panels keep red init 0x00 and red-wins.
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