Sure! These kinds of things are addressed in the Help file, and this one is called Undocking Windows.

If you have a second monitor, you can also undock and move the preview window to that one and leave the main editing window on the current monitor.

Learning to use PD18 on a trial and error basis, I did that once by accident - that is, undock the preview window: what does this icon do? click and the result covered a load of the library items which had expanded to a full screen on the only display I have, overlaid by the undocked window (many library items thus hidden friom view). And I had a hell of a time figuring out what I'd done, and restoring to normal.

Undocking is basically about how to use more than one monitor. And the usual practice would be to put the libraries and timeline on one and the preview on the other. However, most - 99%plus - users don't have more than one monitor, and all of them at some time would like to see what their efforts actually look like on a normal screen size.

What I was expecting was an icon toggle somewhere (I'd missed it) which put the preview display on a full screen with some basic controls (play, stop, plus the reverse toggle - go back to normal view). Other editors have that feature, but clearly PD18 doesn't have it - and is poorer for the lack.

I'll post a feedback item.

I actually disagree about the utility of having an undocked window even with a single monitor, but I may have led you astray when you asked if there was a toggle between the normal window and a full screen preview.

In fact there is, and all you need to do is double click on the preview window to bring up a full screen view with minimal controls at the bottom.

The controls disappear after a couple seconds if you move the cursor away from them, and they'll reappear when you move the cursor again. Click the X at the right edge of the control panel to close the window and return to the editor.

Note that the preview resolution will be the same as during normal editing, so you may need to change it fro the main editor window to get the clearest picture.

Yep, that works and is what I was looking for.
I checked thru the User Guide for this - and there doesn't seem to be anything it: lots of other stuff, but not this. And since most users RFM only as a last resort (mea culpa) an indicator/icon under the preview window to do it seems like a more user friendly way to do it.