quick way to turn a selection into a new file?

Here’s one idea that has the potential of streamlining through actions or keyboard shortcuts:

1. Choose Layer > New > Layer Via Copy. The selection is now a new layer which is now the selected layer in the Layers panel.

2. Choose Layer > Duplicate Layer, and in the dialog box that opens, choose New from the Document drop-down list, and click OK. This creates a new document from that layer. (Optional, while you’re in Duplicate Layer: You can name the layer and the new document.)

You get a new, untitled document with just the selection on a transparent background. It is the size of the entire canvas of the document it came from, so if you only want it to be the size of the selection, add this step:

3. Choose Image > Trim, select Transparent Pixels, click OK.

In the Actions panel it’s easy to record those three steps so that you can run the whole thing in one click, especially if you put the Actions panel into Button Mode. You can also assign a keyboard shortcut to the action, but an action shortcut has to use a function key.

If you want to run this sequence yourself with keyboard shortcuts:

There is already a keyboard shortcut for Layer Via Copy, so press that. There are no default keyboard shortcuts for Duplicate Layer or Trim, so use Edit > Keyboard Shortcuts to add whatever shortcuts you want for those. Then you will have three shortcuts you can press in order. The only speed bump is that while in the Duplicate Layer dialog box you have to select New for Document, so you either have to do that with the mouse or use Windows dialog box keyboard navigation to do it. (On the Mac, apparently some custom dialog box code does not allow standard macOS keyboard navigation of dialog boxes to work.)

Have you tried recording these steps into an Action?