SecurityError in new canvas

[BertrandTheHealer]

When opening the sandbox or a game while using the new canvas, an alert appears saying "SecurityError: The operation is insecure. line: 5209"

This is not something that Firefox does normally for this kind of error; something on the page or in the userscript is launching an alert in response to the error.
This doesn't happen in the default canvas, so the error itself is caused by something in the new canvas. However, it is possible that Drawception itself causing the alert.
This is a minor inconvenience but it would be nice if (a) the error could be fixed or (b) if alert could be suppressed for this particular error if in fact the alert is launched by ANBT.
This does not affect the usability of the rest of the page.
This happens in Firefox 59 but not the latest version of Chrome.

[grompe]

So, this happens at the time new canvas is overwriting the current webpage?

[BertrandTheHealer]

Correct. As soon as the new canvas finishes loading,