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.
You can bind a mousedown event in your canvas to prevent default behavior.
Something like:
// with jQuery
$( "#canvasId" ).mousedown(function(event){
event.preventDefault();
});
// without jQuery
document.getElementById( "canvasId" ).onmousedown = function(event){
event.preventDefault();
};
Here is the updated fiddle: http://jsfiddle.net/MZ9Xm/1/
You will need to test this to see if there is some side effect in what you are doing.