Hello 
In apps is a pretty common way to use grip on vertically slides elements. Discourse’s new update contains slide gesture to close modal on mobile. I think it would be a great addition to drop the X close button and add a grip in the middle of the top to feels more like an app.
An example: Facebook mobile app (android)
Or on mobile OS’s for example Android when you click the upload.
An other way is place the grip outside the modal which also feels nice.
Discourse composer is also contains a grip which indicate very well there is a dragging point to resize the composer.
Thanks 