Strange performance issue on mouse region set

[Andrea Cardaci]

Well, this is weird... I noticed that under certain circumstances the
redraw performance get sensibly worse. In particular there is some
noticeable lag when I set the selection with the mouse: click and hold +
quickly move the mouse un and down, the highlighted region cannot keep up
with the pointer.

The funny thing is that it only happens when all the following are true:

- the mode line has NOT the box around text attribute set;
- the menu bar is hidden;
- the tool bar is hidden.

Here's how I can reproduce that:

1. open a big enough file in order to allow the aforementioned mouse
motion, e.g.:

        emacs -Q .emacs

2. make the frame fullscreen (F11) to increase the redraw area and thus to
emphasize the effect;

3. so far the lag is not present and everything is quite snappy;

4. evaluate:

        (custom-set-faces '(mode-line ((t (:box nil)))))
        (custom-set-variables '(menu-bar-mode nil))
        (custom-set-variables '(tool-bar-mode nil))

5. the lag appears.

I experience equivalent results with the Lucid toolkit.

I'm using the current master branch
(4daca38d5c673c5b6862e10cfade9559852cce12) and I'm using Linux.
Please let me know if you need additional details.

Thanks,
Andrea