More of an FYI than anything else: prior to QPR2, windowing was active by default on the Pixel Tablet (the top-of-screen pill). Now, that seems to be gated behind developer options in a somewhat counterintuitively-labeled option. Under Window Management, you need to toggle "Enable desktop experience features (Enable desktop windowing on secondary displays)." But of course we're not talking about secondary displays; we're talking about the tablet itself. Regardless, it works.
You can turn off developer options and keep this setting on.
I very rarely use the feature so didn't even know it was gone at first but figured I'd drop a post here for the tens of people who have a Pixel Tablet in the first place and care about this feature.