This works all right in practice. Aesthetically, I dislike the black background that Microsoft shows on the top of the screen while the rest of it is pulled down. It feels like you've fallen off the home screen. (Show the background instead!) But it works, including the ability to pull down the top drawer of quick-action items.
A potpourri of icon changes
Microsoft also made a number of random changes to the new build's UI. The PIN pad is transparent, for example. The quick-action shortcuts on the top of the screen differ somewhat from the last build, too--they're flatter, and boast a different font. I noticed that when I tapped the back arrow to load a previous app, a grid of apps appeared, rather than a horizontal queue. Microsoft also tucked a new background behind my home screen, of a swimmer cutting though the water.
Because I loaded the new build of Windows 10 onto a new phone, I wasn't able to draw direct comparisons to the performance of the new build, one over the other. It's quite a mixed bag, however. Cortana's responses were extremely snappy, to the point that I wondered if frequent requests were cached on the phone. But every time I jumped back to the home screen, I saw a "loading" icon for a second or two. I don't want to see that, ever.
My Surface Pro 3 also failed to recognize the Windows Phone when connected to it. The phone's OneDrive app doesn't seem to quite work, either, so it's a bit of a pain getting photos off the device. Twitter's app repeatedly crashed, as well.
We can chalk all that up to beta software. Still, the clock's ticking. If the reports are correct, we have about three and a half months, max, before Windows Mobile 10 rolls out. I'm enthused by how Cortana looks. The next step is applying the same sort of rigor to the remainder of the operating system and its apps.
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