Yes, as I said, qemu is able to change its window size when you change the
resolution so this should not be a problem. Worst case is you get a window
larger than your screen, or maybe qemu itself would crash. But not the Haiku
kernel.
Maybe there is a problem with our code (for example, the mailbox is not
mapped in memory at the expected place), or maybe qemu does not implement the
mailboxes properly. You have to debug at a low level (asm instructions)
exactly what is happening and why it is not working as expected.