You are using Email to Case Premium, and would like to know if Chrome Extensions such as Grammarly are supported on the New Comment page.
Unfortunately, there are limitations with Grammarly that prevent us from fully supporting this extension in Email to Case Premium.
When the Enhanced Comment Editor feature is enabled, this exposes a special rich text editor on the New Comment page that utilizes TinyMCE, an open source project licensed under LGPL. Using TinyMCE's rich text editor on the Lightning New Comment page requires the use of an iFrame, and at this time, the use of Grammarly is not supported within iFrames.
Grammarly does not explicitly mention the iFrame limitation in their documentation, but here are a few online resources where other Grammarly users make mention of this limitation:
The New Comment page designed for Salesforce Classic does not utilize an iFrame. While we strongly advise the use of Email to Case Premium inside of Lightning Experience, Salesforce Classic users should not be impacted by this limitation.