I am watching a tutorial in which the instructor added a shape to an existing layer, but I don't know how he did it.
His cursor was an arrow, he began dragging from off the canvas (top left), then dragged to the bottom right of the below image. As a result, the pink rounded rectangle that is selected in the image filled up the bottom corners of the image.
I know that holding Shift will add to the current layer, but a) His cursor was definitely an arrow rather than a big and small cross shape, so I don't think he did that, b) Keeping Shift held ALSO constrained the proportions of the new shape that I was adding to the current layer. I even selected the "Unconstrained" from the options but it still comes out as a square...