how and why does keyboard input unhighlight the mouse-selected region?

[Kevin Rodgers]

If I select the region with the mouse (with transient mark mode
disabled), it is highlighted using the `region' face.  I think that is
implemented via `mouse-drag-overlay' in lisp/mouse.el.  Then any
subsequent keyboard input -- even typing a prefix key like `C-x' --
causes it to be unhighlighted; `C-h v' shows

| mouse-drag-overlay's value is
| #<overlay in no buffer>

I don't see any references to mouse-drag-overlay in the src/*.c files,
or anywhere outside of lisp/mouse.el and lisp/mouse-sel.el for that
matter.  So how is the unhighlighting of the selected region
implemented?  And why is that the default behavior -- no command has
been executed, certainly not any command that modifies the buffer or
alters the region.  Finally, can the unhighlighting be disabled?

--
Kevin Rodgers