What is the fastest way to make a new image out of the selection or selected layer? I thought I remembered it was as easy as dragging it to the top where the tabs for different images is but can't remember.
Make a selection with a selection tool, or use Ctrl+A to select the entire image on the selected layer
Copy it using Ctrl+C
Do File > Create > From clipboard, or use the short cut Shift+Ctrl+V
It is also possible to create a new image from a layer by clicking and dragging a layer from the layers panel, and drop it onto the toolbar.
The instructions work perfectly, and the creation of the new file from layer is near-instantaneous, but I just encountered enough trouble regarding precise communication as to cause me to create this account, log in, and advise that, where above, per Billy Kerr and Scrampus, you see "toolBAR" you might do well to instead read "toolBOX".
I spent far too long futzing around looking for a "toolBAR", even going so far as to futilly Google for it, and of course it's all my fault, for being thick-headed, but... I have my toolBOX as a separated "window" and it's long and tall, with the variety of tools up at its top end, the color choice overlapping squares down beneath that, and below that, the particulars of whatever tool has been invoked by clicking on it up in the top end.
From the Layers Panel, the layer you want to create a new image from must be dragged into the top end of the toolBOX, at or above the level of the color choice overlapping squares, and when I do it correctly, I get a bit of thin hard-white highlighting around the color-choice area. Done wrong, done too low, down where the particulars of the invoked tool are found, at or below the "Tool Options" divider bar, I get a thin hard-white highlight around that lower portion of the toolbox, but when I drop my chosen layer into it, it bounces back over to the Layers Panel and nothing happens. So you have to watch out for all of that, lest you burn far too much time trying to figure it out. GIMP 2.10.30 running in Linux Mint Mate.