You may also wish to change the popup where the user types the text of the bug report. Currently, it's just a window.prompt - and it would be far better as a fancy text area in a more native dialogue to your app.
You could even extend out interactions with the html2canvas library, such as making it display the screenshot to the user and letting them draw circles/squares/arrows on it before submitted it to our api, or highlighting/blacking out certain elements (similar to this demo).
The underlying problem is we want to allow unauthenticated users to create cards (bug reports) in your Trello account. Trello grants an api token that allows access to all of a user's boards/lists - which is far too permissive, we can't use that in a frontend app.
Finally, one last thing you might try... there are two different versions of the html2canvas library. Version 0.4.1 is what we use by default, as it's hosted in Cloudflare's CDN. I have noticed that version 1.0.0 renders certain things better - but unfortunately it's not available in a CDN. In the code, search for the bwc_html2canvas_version and swap it from 0.4.1 to 1.0.0 and see if that works better for you. If it does, consider downloading the 1.0.0 version of the html2canvas and hosting it on your server and using it (the "1.0.0" in the script is from a demo server with limited bandwidth, sufficient for testing but not for production).