RRPyttriumregarding what problem exactly?
I ask because there are lots of issues with windowed mode so I don't know which one you're explicitly talking about
Forced vsync and/or other input lag causing crap, dualmonitor stuttering
Oh, that.
I was actually really sensitive to this issue back in 7, and I made a couple of long, detailed posts about my findings back then here on TFTV.
For what it's worth, I play in borderless windowed for almost every game in Win10, and while it's not perfect, it's FAR better for me than it was in 7 (with aero on). I'm not sure what they've really improved but it does actually feel like it's 144Hz.
Playing a video on another monitor while I play games on my primary 144Hz one will reduce the framerate on the primary, but that actually depends on how demanding the video is (resolution/what browser/what type of player it uses, flash or html5) so I think that's just misprioritized system resources causing issues, rather than actual vsync bugs.