Version: 55.0.2882.0 (Official Build) canary (64-bit)
OS: Mac OS X 10.11.6 (15G1004)

What steps will reproduce the problem?
(1) Open http://output.jsbin.com/qucahuteve/
(2) Open DevTools console
(3) Enable responsive device mode and resize window

What is the expected output?
The width of the ResizeObserver event should be changing in width

What do you see instead?
Width always reports 980px below a certain width.

The test case works fine when resizing the actual window


Please use labels and text to provide additional information.