in win 10, maximized window won't drag

Don't maximize it and then you can drag its edges to make it full size if you want that - and then you can move it by grabbing the title bar.

Click the little square icon to the left of the top right x on the title bar.

That's not a fix.

Comon, I could drag a maximized window for the 2 years I've had the laptop, until a few days ago. Something changed and I want to set it back.

I could drag a maximized window 

Mike is right.  If the window is maximized the window's window menu will prevent you from moving or resizing it and that has always been the case.   Use Alt-Space to see this.  Restore, Minimize and Close are your only options in this state.  So almost certainly you would have been dragging a maximally sized window.  In that case everything except Restore would have been available--a sign that the window is in its normal mode.

FYI

Robert Aldwinckle

Richard.

You certainly respond with some implied authority. So in your vast and insightful vision of all beings using a computer, I've not clicked the little square icon at the top any window but have always been dragging the corners to make it appear maximized.

Well shucks, I've forgotten this little exercise, which I must have done countless times. Wow!! Scratching my head. EXCUSE ME!

But really, there's some disconnect here. Like I said, I maximize some windows, but did not have to click the double-box icon to reduce the size. All I had to do was drag from the title bar and the window reduced to the previous non-maximized size.

Comon, if you don't want to explain or don't know, just say so.

The Meat

I maximize some windows, but did not have to click the double-box icon to reduce the size. All I had to do was drag from the title bar and the window reduced to the previous non-maximized size.

You can do that using the Snap windows feature but check out the window menu when you do that.  E.g. press Win+CursorUp ;  that snaps the window to Maximized.   Press Alt-Space to see that only the menu items I mentioned before for a maximized window are available.   When you are using the Snap windows feature with your pointer you need to drag it up first and observe the ghost maximized frame before clicking to make it snap to Maximized mode.  An analogous procedure is needed when you want to snap a window left or right.  Then I think the use of the pointer for a half screen may be clearer when snapping a window into a new mode and undoing the snap.

So, to clarify the original discussion, you weren't really dragging a maximized window you were moving your pointer in a way that looked as if you were but in fact were only doing that to change the window's mode and size.

FYI

Robert