Ah, nice!

I noticed that heifsave.c has almost the same issue with stride:

    for (y = 0; y < area->height; y++) {
        /* Y in page. 
         */ 
        int page = (area->top + y) / heif->page_height;
        int line = (area->top + y) % heif->page_height;
        VipsPel *p = VIPS_REGION_ADDR(region, 0, area->top + y);
        VipsPel *q = heif->data + line * heif->stride;

This feels like an error in heif_image_get_plane(), though I suppose it'd be tricky for libheif to change their API.

Ah, nice spot. heifsave.c is limited to a maximum image size of 16384x16384 (after PR #3513), but let me fix that occurrence too.