Javascript画布显示错误

Question

I am working on an offshoot of Snap! which is a visual, javascript IDE (I know, I know). The newest release looks fine to me, as it does for the two others working on this. However, one person who uses this code gets a big white box over the black background on some of the projects.

I cannot reproduce the error on any other computer. I have tried Chrome versions 38-45 (they were using 45), tried running with and without their extensions, cleared cache, checked their settings especially hardware optimization, restarted and checked the OS (windows 7 up to date), and closed all other programs that were running. I have gone over the code half a dozen times, and while there must be something that I missed, I cannot find the source.

Question 1. When you open (chrome only) https://dev1.csdt.rpi.edu/projects/293/run , does anyone else see something in the top right corner besides a bunch of "beads" on a black, numbered background? Specifically a white box.

Question 2. Does anyone have any guesses as to what I might have missed?

Thank you in advance.

Answer 1

Thanks to Octopus and Damon Smith I set up a virtual box running Ubuntu and was able to reproduce the error, and then solve it.

Specifically the error was that in some versions of chrome, especially Linux chrome, the default canvas is not transparent as it is on Windows. So when one canvas was created on top of another one, it would create a white box. Because we are using two canvases, one for 2D and one for 3D, there is a white box over the 2D background image.

Thus, my solution is to force all 3D canvases to have a transparent background. This was done by modifying the code as bellow:

StageMorph.prototype.get3dCanvas = function () {
    if (!this.canvas3D) {
         this.canvas3D = newCanvas(this.dimensions);
         context = this.canvas3D.getContext("2d");
         context.fillRect(0, 0, this.dimensions.x, this.dimensions.y);
         context.fillStyle="rgba(0, 0, 200, 0)";
         context.fill();
    }
    return this.canvas3D;
};

I don't know how this problem cropped up on windows, but I am glad it did, so that we could troubleshoot this for all users. Once again, thanks for all the help.