I don't have PSE 21, but this might work for you if you want to keep your PSD/TIF with layers.
First, make sure you don't have any smart objects. This won't work if you haven't rasterized them. You can select every layer in your layers panel and then right click and choose RASTERIZE LAYERS.
Second, click anywhere on the layout canvas and press CTRL+A. This will select all the layers.
Third, go to the toolbar at the top and choose IMAGE>CROP. You won't see any visible change at this point. PRESS CTRL+D to remove the marching ants around the canvas.
Fourth, you can go to one of the layers that you know was outside the canvas and you should see that the extra pixels are now gone.
I know that's not one shortcut but a series of steps. Maybe someone will jump in that knows PSE better.