UX for Reference Layers is a little weird and barebones right now. Reference layers can currently be transformed, but only if:

1. You have the Move (V) tool selected.
2. You place your cursor over the bottom-left corner of the reference layer's contents.

There are no handles that suggest this behavior even exists to the user, which frankly makes it kind of clunky.

Weirder, attempts to use Edit > Transform are met with an error message, even though transform handles and selection marquee do appear (though they don't match the reference layer's current scale).

Proposing one of two solutions to make this more intuitive.

1. Allow reference layers to be transformable via Edit > Transform. Limit transforms to scale and position, as this would effectively retain current capabilities.
2. When a reference layer is selected, automatically show transform handles. It should be safe for this behavior to occur regardless of which tool is active, as it's not possible to draw on or select portions of a reference layer anyway.