Fix open panel detection when scrolling

With touch devices, scrolling may get negative values (e.g., when
scrolling beyond the page limits). That kind fo scroll was triggering
the aplication of the "panel-oppen" class producing unintended UI changes.
With this patchset the class is only applied when the panel is really opened.

Change-Id: I183d8c725373ab29802650b53e26fd4820dbf499