Hitfilm 2017 has more problems with the buggy VFR output from Handbrake than Hitfilm 4 used to. That seemed to read the intended framerate, take that as Constant and ignore the variations and it worked out fine. While that was actually a bug, it cancelled out the Handbrake bug.
tl;dr; Hitfilm's new way of handling VFR files shows up a Handbrake bug because when it incorrectly rounds up 23.976 to 24fps: video and audio lose sync.
The fact that people have been using Handbrake to transcode files before means that if they do so now, or open HF4 projects with Handbraked clips in them: they're in for problems with video and audio sync.
Thanks to @NormanPCN who confirmed it wasn't just on my PC that Handbrake was playing up.
This was using a clip from Tobias's (Surfaced Studio on YouTube) recent tutorial on Hitfilm tracking, where he evidently also discovered the Text bug when you can't actually type > or < into the text box and provided a .PNG file for the arrow. You can cut'n'paste > or < in there though.
As his provided clip performed brilliantly in Hitfilm and scrubbed and played like velvet, I wondered what had been used to produce it and also if it could be improved even further in Handbrake.
After trying many, many settings that only ever produced VFR output, I discovered that even doing absolutely nothing in Handbrake (not even changing the Main Profile, as in the video) would always produce VFR output. And that output did not perform well in Hitfilm. Playing with other fast decode settings etc. only made the file bigger and it still didn't perform as well as the original. I want whatever Tobias used to export that clip.
So with Handbrake being completely useless: I wondered how Hitfilm coped with the CFR>VFR output that Handbrake insisted on providing. Answer: Not as well as you'd hope.
Note: The two clips have the same average bitrate, are nearly identical in size, but one plays as smooth as butter, the other...doesn't, so it's not just bitrates and files sizes and profiles that make a difference to performance. However the alphabet salad of settings that Handbrake provided did nothing useful.