The way "open folder" is implemented is to set window.location.href with the folder query variable.

Manually editing the URL, manually calling window.location.href, or even just reloading the page can also trigger the hang. It may take several attempts or waiting a bit (at least 7-8 seconds seems to be the ticket) before doing the navigation. The resulting request will be pending forever.

This is on Coder v0.8.1-devel+6e63487b running in an image with the coder template. It happens on both Firefox and Chromium.

I was not able to reproduce it outside of Coder.