Change canvas size

[SF]

GIMP Version: 2.8.14
Operating System: Windows
GIMP Experience: New User

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Whenever I resize the canvas and click okay, the canvas is filled with the background color. I've specifically checked the pop-up states fill with transparency, but it still does it. I'm on a new computer, so may be a setting I'm missing. Can you please help?

[dinasset]

Before resizing the canvas:
- Layer > Transparency > Add Alpha Channel
then resize canvas.