GIMP Version: 2.8.14
Operating System: Windows
GIMP Experience: Intermediate Level

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Why is this happening when I rotation a single layer image with no alpha channel?

Because you have rotated the layer, but now the canvas is the wrong shape/size.

Either change the shape of the canvas after rotation, Image >> Fit Canvas to Layers

or rotate the image, not the layer, Image >> Transform >> Rotate

Kevin

Makes sense, in a way, I guess. The image is a png, and does not have multiple layers. Interesting that the rotate tool treats the image and the canvas as two separate things. Odd to me that the canvas doesn't just follow the image.

Thanks!

The canvas is predetermined when created (New) and placed exactly as determined when opened. So it is given a "background" layer. Which is why, when rotating, you need to treat that rotation with the Image menu, if you want the entire image and canvas rotating as a whole. It is a nuance, but easily remedied.