The header-doc for get_color_channel_data_at in struct JxlChunkedFrameInputSource says the following:

Moreover, xsize and ysize will be at most 2048.

This is incorrect. If JxlEncoderProcessOutput is used (i.e. not streamed encoding), the full buffer will be requested. There is no mention of this anywhere in the header-doc. I can see the following comment in the code when getting called for the full buffer:

    // The input callbacks are only guaranteed to be available during frame
    // encoding when both the input and the output is streaming. In all other
    // cases we need to create an internal copy of the frame data.

The header-doc should really call out that doing chunked encoding is pointless unless you also do streamed encoding.