Here's a fiddle: http://jsfiddle.net/MZ9Xm/
Note: The following occurs in Chrome 22.0.1221.1, but not in Firefox 14.0.1. [Ubuntu linux]
Move your mouse to the top canvas and press and hold the mouse button. Drag the mouse, and the cursor will change to a text-selection mouse cursor (I-bar). This does not happen if there are no other elements on the page.
I've messed around with setting user-selection to none, but have not had any luck.