dialer-app sometimes stops rendering views

I noticed that sometimes dialer-app looks stuck, but in fact it's still running and responding to touch events.

After some debug I found one way to reproduce it, but only in greeter mode. I got this same bug in normal mode many times before, but didn't find any reliable way to reproduce it. (video attached)

This is the backtrace while the app looks stuck: http://pastebin.ubuntu.com/13210479/

Thread 2 is in general the rendering thread, but not sure if that's always the case. If so, it is stuck in a mutex.

If we stop interacting with the app for some seconds, and then tap anywhere on the screen, the view gets then updated and everything is back to normal.

An additional information is that every time the bug is reproduced, I see the following message in the unity8 logs:
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qtmir.surfaces: MirSurfaceItem::dropPendingBuffer() surface = qtmir::MirSurface(0x115f0a8) buffer dropped. 0 left.
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