UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36

Steps to reproduce the problem:
1. Open or find a window with a vertical or horizontal scrollbar (Copy-paste if needed: document.write('<div style="width: 200%; height: 200%">&nbsp;</div>'); )
2. Click down on the scrollbar track, move the mouse away from the scrollbar area (but not enough to break drag lock), then scroll via the mouse wheel while holding the mouse still. Observe that drag lock is retained, and moving the mouse will also scroll the page, since the button is still held down.
3. Repeat the above, except keep the mouse inside the scrollbar area when you scroll with the mouse wheel. Observe that drag-lock is broken, the scrollbar reverts from "armed" (pressed-down) state to "hover" state, and moving the mouse (which should produce drag events on the scrollbar since the mouse button is still down) no longer occurs.

PS: this test may not work with two-finger scroll systems, simply because they may generate button-up or other interfering events when you lift your second finger off the trackpad.

What is the expected behavior?
That's actually tricky. That #1 and #2 differ is a UI consistency bug, but the question becomes, which behavior is the most intuitive?

On the one hand, you can listen to the fact that the mouse is dragging the scrollbar, and retain drag lock.

On the other hand, scroll too far away using the wheel and the next mouse movement will yank the page a jarring/disorientating distance if you're not mentally tracking (and understand) what you're doing.

In my case, I was navigating one of Wikipedia's many wide tables using a traditional vertical-only scrolling mouse, dragging the horizontal scrollbar to scroll horizontally, and using the scrollwheel to scroll vertically.

So that's at least one use-case that justifies keeping drag lock active as long as I have the button down.

What went wrong?
See other comments

Did this work before? N/A 

Chrome version: 51.0.2704.79  Channel: dev
OS Version: 51.0.2704.79
Flash Version: Shockwave Flash 22.0 r0