Chrome Version: 69.0.3497.92 (Official Build) (64-bit)
OS: Linux

What steps will reproduce the problem?
(1) Enable "Experimental Web Platform features" flag for VTTRegion support.
(2) Create a TextTrack on a video element.
(3) Create a VTTCue with a VTTRegion applied.

See: https://storage.googleapis.com/shaka-demo-assets/_bugs/firefox-vttregion-style/text.html

What is the expected result?
Cue should appear at the bottom of the region, similar to how the cue appears at the bottom of the video with no regions.

What happens instead?
The cue appears near the top of the region.

If a second cue is added, the first doesn't move and instead the second appears outside the region below the video (only the top can be seen because it is clipped).  In Firefox, the cues appear at the bottom and stack correctly.