bug#5557: <left-margin> <double-wheel-down> is undefined

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Mon, 17 Aug 2020 13:30:37 +0000
> Cc: rudalics@gmx.at, larsi@gnus.org, 5557@debbugs.gnu.org, 
>       lennart.borgman@gmail.com
> 
> Please find attached a new patch which should fix the issues you pointed
> out.

Thanks.

> +*** Mouse wheel scrolling less sensitive to positioning.

I'd suggest

 Mouse-wheel scrolling now works on more parts of frame's display.

> +When using 'mwheel-mode' (enabled by default), the mouse wheel will
> +now scroll also when the mouse cursor is on the fringe, margin, header
> +line, and mode line.

The code also arranges for the scroll bars, no?  Also, please use
"fringes" and "margins", in plural.