Gimp: adding new content to the bottom of an image

I've opened a PNG image in Gimp, and I want to add more content to the bottom of the image, creating a taller image.

My Gimp skills are pretty basic, and the only approach I've found that seems to be on the right track ultimately doesn't work. I go to the Layers -> Layer Boundary Size menu item, click the chain icon so I can vary the dimensions independently, increase the height by (say) 100 pixels, and click the Resize button. Back in the main window the image boundaries seem to have increased, but when I try to, say, add some new text into the newly expanded area, or copy and paste a region from the original image into the new area, it's invisible.

How can I add new space to the bottom of an image so that I can add new content to it?

Solution 1:

You have to go to Image > Canvas size and unlock the chain icon that keeps the height and the size proportional when you change either of them.

Solution 2:

Image > Canvas Size

Solution 3:

On the canvas right click Image->Canvas Size...