mako doesn't work when multiple notifications are received

[inclement]

A single mako notification displays fine and can be dismissed by clicking it, but if a second one is opened (e.g. by successive notify-send calls) no new popup appears and the existing one can no longer be dismissed.

Sway works fine on the same system and mako version, so it must be something we're doing wrong.

From quick investigation I'm not sure mako is even trying to create a new layer surface - Vivarium would log if it did. Perhaps we've missed some response to a previous mako surface event that would make it behave correctly?