Using the Crop tool to enlarge the canvas

[Krikor]

(06-17-2019, 06:01 PM)Ofnuts Wrote: 10 years of Gimp usage, and never noticed the Allow growing setting in the Crop tool. When you set it, you can use the crop tool to make the canvas/layer bigger.

I've been looking at this option - Allow Growing. When selected, provides the menu: Fill With, which provides options such as FB, BG, Pattern, White and Transparency.

However, I could not get the fill with any of these options, just the growth of the layer. 
What should be done to fill the enlarged area with any of these options? 

Thx.