Long-term it would be nicer to have the synchronization directly in KWayland,

In terms of long term, I want to have explicit synchronization handled by kwin.
We should expose OutputInterface::done() to be explicit rather than being done implicitly in setBlah. The attempt to make things simpler actually ends up making things a lot harder to do properly.

As for this, I made an alternative approach: https://phabricator.kde.org/D19255
It's basically the same thing, but without checking for differences twice.

I've not tested for the problem you're solving, but it should be the same.

I wrote my comment before you added your last two comments. Mine won't fix the issue for the same reason yours doesn't.
Still maybe makes sense.

But in either case, my comment about it being OutputInteface with the problem not XdgOutputInterface still apply.

In D19253#418085, @davidedmundson wrote:

Long-term it would be nicer to have the synchronization directly in KWayland,

In terms of long term, I want to have explicit synchronization handled by kwin.
We should expose OutputInterface::done() to be explicit rather than being done implicitly in setBlah. The attempt to make things simpler actually ends up making things a lot harder to do properly.

I would say this is a different question. We should indeed expose the done event of a "generic" output, such that not after each property change we send the done event. But that afterwards the xdg_output done event must be sent could be handled by KWayland directly. On the other side if we expose the done event of OutputInterface it's clear when to call the done event of XdgOutputInterface as well. It's just a bit clunky to not do this directly in the library but have the compositor writer keep track what was called already and what not, I think.

As for this, I made an alternative approach: https://phabricator.kde.org/D19255
It's basically the same thing, but without checking for differences twice.

I've not tested for the problem you're solving, but it should be the same.

The description says "Done nothing yet". Does this mean one should wait with testing until you made some further changes to the diff?

The nothing done yet was on regards to testing.

Its ready for testing, but it'll behave the same as yours.