Lag on first prompt buffer invocation (WebKitGTK port)

[aadcg]

On the first invocation per window, exclusively, a blank box is drawn before showing the prompt buffer contents which feels a bit clunky.

It seems to be related to the creation of the WebKit's view (which is is done once and re-used subsequently).

It would be great to either understand if there's a way to go around this expensive call or to make a dummy call so that the user never gets to experience it.