Rendering issue in devtools scope tree

## Problem
We have recently noticed the following rendering issue with the devtools scope tree.


## Environment / Observations

- this started happening a few weeks back.
- seems to happen intermittently although presently it is all the time.
- happens both on my Linux Ubuntu (Ubuntu 18.04.3 LTS) desktop and a colleague's Windows 10.
- Chrome Version 77.0.3865.75 (Official Build) (64-bit)
- Chrome restart does not clear it.
- Playing with docking position does not change anything.
- Sometimes I have seen it render some of the vars but not all of them if you resize the devtools scope panel.
- If you disable custom formatters in settings, it renders fine but alas back to raw JS data structures.
- Rendering same ds in console via `js/console.log` does the right thing and expandable tree renders perfectly.

This might be a cljs-devtools issue I see latest 0.9.10 was released over a year ago. I'm not sure but did dirac embed the devtools and is it possibly running a more up to date version ? (we don't use dirac at the moment, but if installing it instead of cljs-devtools would that solve it ?

The issue names no source file or test; first reproduce the scope-tree rendering problem in Chrome with custom formatters enabled and compare it with console rendering. Done means variables render completely and correctly in the scope tree without disabling custom formatters, including after resizing the panel.

索引モデルが issue の本文から書いたものです。