If you are sensitive to get things pixel-perfect (like me), I can recommend not to use window.zoomLevel. We do everything correct on our side, paint the <div>s at integer coordinates, etc. But by setting window.zoomLevel to -2 I think Chromium interprets that as 80% and begins multiplying all our nice integers with 0.8. This causes the nice integers to become floats and it is inevitable that they get rounding errors.
I recommend to stick with customizing fontSize and lineHeight. I wonder what makes you want to change window.zoomLevel to begin with?