Link
They've been hinting for years that they're going to make a handheld and never do, but swapping to a handheld console would allow Microsoft to start the next gen while not starting the next gen and give them the ability to jettison the Series consoles without fully abandoning them.
Rumours from a year or so ago were that Microsoft would with with NVidia on their next console. That rumour could have been referring to a handheld, as Nvidia pretty much own the handheld market.
Microsoft's surface is a pretty snazzy tablet. Rumours from donkeys ago said that the next Xbox will be designed and made by the surface team.
I have no links for those rumours but they are/were floating around gaming sites a while back.