I stumbled across a problem that might or might not become an eZ ui problem.

I am using Chrome Canary (59.0.30.70.0) as dev browser and it seems they somehow change behavior on z-index calculation. Content tree second level will get "cut off",manually setting z-index on .ez-menu-container fixes the problem.

I couldnt find relevant entries in the chrome release notes and I am aware that its a canary build.
However if that behavior goes into stable branch with the next chrome update content tree will get unusable