Both the extension and the component were both passing in numbers like 286.6499938964844 but they were behind rounded differently or something?
It seems to me that the extension was flooring the numbers somewhere and the component wasn't flooring or rounding the numbers anywhere.
The two images would start off being the same but then would progressively become more different. See image diff below
{F5955989}
If we round the region before drawing the snapshot the resulting images will match.
Sometimes there is some offset difference (only 1 px difference) because the component now `round`s and I think the extensions `floor`s. `286.6499938964844` would be `286` for the extension and `287` for the component.
Differential Revision: https://phabricator.services.mozilla.com/D199059
a145338e1e