Chrome Version: 66.0.3359.137 (Official Build) beta (64-bit)
OS: Chrome OS (Chromebook Pixel 2015)

What steps will reproduce the problem?
(1) Attach an external display at a 1x device scale factor (mine is a 1080p desktop monitor).
(2) Move a Chrome window to the external monitor.
(3) Trigger the cast UI, for example by clicking the cast button on Google Play Music.
(4) Observe that instead of targeting the UI element under the cursor, the mouse targets things at twice the x- and y-coordinate, relative to the top-left corner. (For example, to click "Stop" in the lower-right corner, I must click in the center of the popup.)

What is the expected result?
Hit testing works normally.

What happens instead?
See above.