Implementations:
Mozilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1272409/
WebKit https://bugs.webkit.org/show_bug.cgi?id=157743/
Microsoft https://developer.microsoft.com/en-us/microsoft-edge/platform/status/resizeobserver/
Performant resize detector based upon scroll events https://github.com/wnr/element-resize-detector/ #44
Good diagram showing all the different height values, why we picked clientWidth: https://msdn.microsoft.com/en-us/library/hh781509(VS.85).aspx