Marking with mouse does not highlight region

[Arash Esbati]

Hi all,

I am observing a strange behavior: Emacs does not highlight region when
marked with mouse during the first mark process.  The region is
highlighted when I release the mouse button.  After that, I can hold the
mouse button and mark, region is highlighted directly.

I started Emacs with `emacs -Q' and tried it with `C-h f forward-sexp'.
The screenshot below shows what happens in the very same help buffer:
Above, the region is marked, but not highlighted, below, I released the
mouse button and see the highlighting.

emacs.jpg
Description: JPEG image

I'm using Emacs 25.0.95 on Win7 64bit.  24.5 does not show this
behavior.  Any idea?

Best, Arash