Ok try explain it clearly, sorry for my english, I'm french first :P
I found the solution to fix it, but it's on frame code, that only you could permenently fix :) let me explain...
So I dont have any .capx for that, cause the probleme is in integration of canvas in my html. So first, here a example of my html and style, minimum requirement to reproduce my problem:
<canvas id="c2canvas" width="1000" height="500"></canvas>
border:1px solid #DE0000;
border:1px solid #000000;
The result of this set canvas overlay top at "200px"... It's wrong cause it's supposed to be "0px" due to the position:relative; in section tag. So the overlay canvas dont position it correctly in that kind of situation. Yea, I can move my canvas tag out of there, but it broke my semantic css...
I looked in c2runtime.js to find a solution, and I just found this function:
runtimeProto.positionOverlayCanvas = function()
var overlay_position = jQuery(this.canvas).offset();
overlay_position.position = "absolute";
If I replace ".offset();" by ".position();", which retrieves the current position relative to the offset parent, it solve the problem!
For now I fix it manualy after each export, but I'm pretty sure you could fix this quickly.
I hope it's my explanations are ok and clear! Tell me if not.