This is my best guess.

• Get an instance of the canvas
• Perform your mutations to the canvas the same way you would in normal javascript
• Serialize the canvas and compare it to a snapshot.

@frazboyz I got the instance but then the actions that I want to do on it is not possible.
Because no command is working on the canvas.

Try something like this.

cy.get('canvas').then(canvas => {
    const element = canvas.get(0);
   // Mutate the canvas using standard javascript, if you need to use another library, you will need to add that to your cypress project.
   return canvas;
});

I would also suggest reading the Cypress documentation further.

If you have any other questions, please use https://github.com/cypress-io/cypress/discussions