[Xrandr][dual-head] Is it possible to disallow mouse to enter invisible area?

[Wolfram Nyaa~]

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Hi, all!

I'm using Gentoo Linux with X.org server 1.8.0 with xf86-video-ati
6.13.0. KMS is enabled.

I've two displays with different resoluitons. They are merged in one big
virtual desktop using the following entries in xorg.conf:

Section "Monitor"
    Identifier   "Dell 2709W"
    Option       "DPMS" "true"
    Option       "Enable" "true"
    Option       "Primary" "true"
EndSection

Section "Monitor"
    Identifier  "Samsung 172T"
    Option      "DPMS" "true"
    Option      "Enable" "true"
    Option      "LeftOf" "Dell 2709W"
EndSection

So I've big invisible area on this desktop, like on this diagram:
+----------+---------------+
| Screen 1 |               |
|          |    Screen 2   |
|----------+               |
|Invisible |               |
+----------+---------------+

All working fine, except one issue: it's allowed to mouse to enter
the invisible area. It's very annoying behavior.

The best behavior I've ever seen was with these two displays
configured as independent X displays: mouse cursor jumps into
screen 1 when I'm trying to move it to invisible area from screen 2
and vise versa.

Could somebody give me any directions to get this behavior with virtual
desktop?

I'm ready to try writing patches for XRandr if it's not possible
currently. So, any hints related to code, which manages mouse cursor
behavior will be useful.

Sorry for possible mistakes in my messages.
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