Make sure users know that a preview is being generated below panel

[JammyDodger]

@debryc

@TZornG

narrow the welcome panel down to 2/3 its width to make the underlying preview pane visible.

Yes, please.

Something like this?

For that I shaved 5% off the width. My CSS is very novice though, so it may not be a genuine solution.

.composer-popup {
    width: calc(45% - 16px);
}