If you are developing a panel that is affected by a small grey rectangular artifact as demonstrated below:
…you can call the following API to hide it.
window.require('photoshop').core.suppressResizeGripper({"type": "panel", "target": <panelId>, "value": true})
where the panelId is the one defined in your manifest.json .
We are looking to make the described a more permanent ‘opt-in’ experience versus the runtime API, but the API is not slated to be removed any time soon.
(On behalf of Heewoo, one of the Ps extensibility engineers.)
Small tip for not having to hardcode the panelID (at least that’s how I did it to begin with):
Depending on your setup, you might be able to import JSON files like
import manifest from '../uxp/manifest.json'
and access it via manifest.entrypoints[0].id in case of a single-panel-plugin (index 0).
This makes it much less prone to errors when changing the ID at a later point.
I am struggling to remove this square from the bottom right corner.
In my index.js I have put
window.require('photoshop').core.suppressResizeGripper(
{
"type": "panel",
"target": "com.jdp.frames",
"value": true
})
“target” has to be your panel id, specified in the Manifest under “entrypoints” 
Thank You, Its all working again now 