Reproducible browser hang in Julia cell

[akdor1154]

Hey all,
I managed to track down the a ( 😄 ) nasty full-browser hang I was seeing when investigating #5469 .
To reproduce:

• Create a new Julia notebook.
• Create a cell, paste in

f(
    [i for i in list])
)

( note the extra close bracket )

• Wait for CodeMirror to kick in (on my local there appears to be a delay until your Julia kernel starts, which takes a while) and highlight your code.
• Position the cursor at the start of "list", and type "f2(a".
• Browser will hang shortly after due to what looks like infinite parser loop.

I can't repro this on the CodeMirror Julia demo which is updated to CM 5.55. However it does happen reproducibly on Jupyter Notebook 6.1 pre, which is on CM 5.51. Tested in Firefox.
So, any chance of an upgrade to 5.55?

Cheers