It seems that the coordinates returned by DOM.getBoxModel are scaled by some inexplicable factor. We should investigate.

-- Reproduction: --
(1) Download repro.py from https://gist.github.com/anonymous/20fc983c3264139ed78aed800119a4bb
(2) $ headless_shell --remote-debugging-port=9222
(3a) $ python $script localhost:9222 desktop
(3b) $ python $script localhost:9222 mobile

-- Outputs: --
(a) Without mobile:

nodeid 1, w: 785, h: 785, content: [0, 0, 785, 0, 785, 785, 0, 785]
nodeid 3, w: 785, h: 13770, content: [0, 0, 785, 0, 785, 13770, 0, 13770]
nodeid 93, w: 785, h: 13770, content: [0, 0, 785, 0, 785, 13770, 0, 13770]

(b) With mobile:

nodeid 1, w: 1175, h: 1175, content: [0, 0, 731, 0, 731, 731, 0, 731]
nodeid 3, w: 1175, h: 13776, content: [0, 0, 731, 0, 731, 8576, 0, 8576]
nodeid 93, w: 1175, h: 13776, content: [0, 0, 731, 0, 731, 8576, 0, 8576]

Width/height of nodes are (likely) good in both (a) and (b). Quad coordinates make sense for (a), but seem to be scaled for (b).

From the values, it looks like the quad coordinates are scaled by a factor - in this case: node coordinates = ~1.6 x quad coordinates. I tried with a different website and got a different factor. But it seems that factor is constant for a single page.