This may be invalid, but I'm reporting it anyway so a decision can be made about whether there's a possible fix or if it should just be documented.
UI Resizables don't work with the canvas tag. Problem is that Resizable adds its handles as child elements, but children of a canvas tag are interpreted as fallback content and not rendered if the browser can successfully render the canvas. A similar problem will occur with any other element that uses its content as a fallback, eg. script, applet, or object, but I can't imagine a situation where someone would want to apply Resizables to any of those. Would it also occur if you try to make an element that doesn't take content (eg. image) Resizable?
A workaround for client code is to wrap the canvas in a div and apply the resizable to the div - you can still draw on the canvas as normal, and use a stop handler on the Resizable to adjust its width/height and rerender it when resized.