Screen Sharing workaround

[from-nibly]

I used to have 3 monitors in this configuration

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one 4k on top and 2 1080p on bottom

those 2 1080p monitors are dying and I think I'm just going to switch to splitting the 1 4k into 1 1080x3840 and 2 1080x1080 screens

I've already got it all set up with bspc creating 3 virtual "monitors" on the 4k screen.

The problem comes in when I'm trying to use firefoxes screensharing and wanting to share just one of those virtual monitors. basically 0 people have a 4k screen and so I typically just show one of my 1080p screens. When they were actual monitors I could just share the whole screen and switch windows back and forth. but now with my virtual monitors i can only share a single window or the whole 4k screen.

is this something that bspc can even do? or does this need to be handled at the xrandr layer?