Mouse coordinates after resizing or scrolling

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Canvas apps often rely heavily on user interaction with the mouse, but when the window is resized, the mouse event coordinates that canvas relies on are likely changed because resizing causes the canvas to be offset in a different position relative to the window. Thus, responsive design requires that the canvas offset position be recalculated when the window is resized – and also recalculated when the window is scrolled.

This code listens for window resizing events and recalculates the offsets used in mouse event handlers:

// variables holding the current canvas offset position
//    relative to the window
var offsetX,offsetY;

// a function to recalculate the canvas offsets
function reOffset(){
    var BB=canvas.getBoundingClientRect();
    offsetX=BB.left;
    offsetY=BB.top;        
}

// listen for window resizing (and scrolling) events
//     and then recalculate the canvas offsets
window.onscroll=function(e){ reOffset(); }
window.onresize=function(e){ reOffset(); }

// example usage of the offsets in a mouse handler
function handleMouseUp(e){
    // use offsetX & offsetY to get the correct mouse position
    mouseX=parseInt(e.clientX-offsetX);
    mouseY=parseInt(e.clientY-offsetY);
    // ...
}

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