Prettier VS Code extension sometimes makes Format Document behave erratically

[jankeromnes]

Description

When I install the Prettier VS Code extension in Theia (Gitpod), Format Document seems to start behaving erratically, sometimes deleting or repeating random sequences of characters.

Here is a screencast: https://youtu.be/VyrfhW9gQ4I

Reproduction Steps

1. Open the shields.io repository in Gitpod: https://gitpod.io/#https://github.com/badges/shields
2. Open any .js file
3. Try "Format Selection" and "Format Document" a few times: nothing strange yet
4. Install the Prettier VS Code extension
5. Try "Format Document" again, e.g. by wrongly formatting some code first: very soon, it starts removing or repeating random characters

OS and Theia version: Mac OS, Firefox Nightly, latest Theia in Gitpod

Diagnostics:

[Eliav2]

it seems that the formatting is not done with prettier no matter what you configure, Someone succeeded in configure and using prettier on a gitpod?