Verified FIXED with 2017.3.0b6 and SteamVR 1.2.2

Steps to reproduce:
1. Download and open attached project "FloatingCanvas.zip"
2. Make sure HTC Vive is set up and connected
3. Make sure VR Mode is on in Player Settings\
4. Play the project

Actual result: Left hand has a cube and canvas as child object. Moving hand in space you can see that the canvas lags behind the cube and the controller

Expected: No dragging/lagging behind

Tested and reproduced on:
Windows 10
Unity 5.4.0b22, 5.4.0f1

Workaround:

A workaround is to modify the SteamVR_UpdatePoses.cs script.. change OnPreCull to LateUpdate. This will make the controllers line up with the UI but they will be slightly more latent, so it is only recommended updating in LateUpdate when you need UI attached to the controllers.

A proper fix for this is scheduled for 5.5 where the controllers will be updated before the UI, bringing latency down.