Thank you for submitting this bug report. Unfortunately after testing on Windows 10 x 64 bit and MacOS 10.14 with Nightly 79.0a1 (20200615214838), Beta 78.0b7 (20200612174529) and Release 77.0.1 (20200602222727), I was not able to reproduce the issue that you described.
When trying to reproduce the problem, I tried with my normal internet speed (around 121.9 Mbps), which didn't allow me to disconnect the internet on time when downloading the extension from grammarly.com. I then tried to throttle my bandwidth by changing the "No Throttling" to "GPRS" from the "Network" console and to download multiple 1 GB files from thinkbroadband.com/download. This allowed me to have a speed of around 31.0 Mbps and to disconnect the internet in time, but the extension download progress bar was not displaying any negative values, but switching to a "Verifying" state.