Slider thumb will only move a short distance when mouse is not slid perfectly horizontally

Grab a thumb and slide it horizontally moving your mouse off the RHS of your browser window. The thumb will only side a short distance.

The same occurs when dragging the mouse along and down a little a small change in height and the thumb will stop moving.

(Tested in chrome on OSX )

Reproduce the slider-thumb drag in Chrome on macOS, including dragging beyond the browser's right edge and slightly downward. Trace the slider's mouse-drag handling to find why movement stops, then verify that horizontal dragging continues in both cases without breaking normal thumb movement.

Written by the indexing model from the issue text.