I've gnome-session-fallback installed and I'm using Gnome Classic desktop with compiz. After updating from 11.10 to Precise (beta) on February 22, my Gnome Classic desktop started acting weird. To make sure that this is not related to any of my custom settings, I created a new test account with no customizations, just selected the Gnome Classic session from login screen and was able to reproduce the problem.
I'm using destop wall (it is the default too) and have been used to have some windows overlapping to two (or more) desktop viewports (screens). But now, with Precise, when ever I move and place a window so it is partially offscreen of the currently visible viewport and then switch focus to some other window (or root window), the partially offscreen window automatically moves so it is fully in the current viewport, snapped to the edge that it was crossing.
Ie, if I move window so part of it is crossing the right hand side of viewport and then focus (either with alt-tab or clicking with mouse) some other window, the window moves left until its right edge is snapped to the right edge of the viewport. This happens on all edges of the viewport.
In addition to that, if a window has crossed the bottom edge of the viewport and then snapped back in to view. Now if I try to move such window to middle of the screen (not touching any edges, nor overlapping any other windows), it will still snap back to the bottom edge of the screen when it looses focus. This happens only after the window has crossed the bottom edge, not with other edges.
Neither of these problems happen when using Unity or Gnome Classic (No Effects) with metacity.
If I use ccsm to disable compiz "Place Windows" plugin, the problem goes away, but all new windows are opened in coordinates (0,0), below the top panel so this is not an usable workaround.
Also, If I disable the "Place Windows" plugin and re-enable it, the windows that have crossed the bottom edge stop snapping to bottom edge until they have recrossed it.
I've tested this with updating precise daily since I first noticed the problem and it is still present.
Current versions of compiz related packages are:
ii compiz 1:0.9.7.0~bzr2995-0ubuntu5 OpenGL window and compositing manager
ii compiz-core 1:0.9.7.0~bzr2995-0ubuntu5 OpenGL window and compositing manager
ii compiz-gnome 1:0.9.7.0~bzr2995-0ubuntu5 OpenGL window and compositing manager - GNOME window decorator
ii compiz-plugins 1:0.9.7.0~bzr2995-0ubuntu5 OpenGL window and compositing manager - plugins
ii compiz-plugins-default 1:0.9.7.0~bzr2995-0ubuntu5 OpenGL window and compositing manager - default plugins
ii compiz-plugins-main 1:0.9.7.0~bzr19-0ubuntu5 Compiz plugins - main collection
ii compiz-plugins-main-default 1:0.9.7.0~bzr19-0ubuntu5 Compiz plugins - main default collection
ii compizconfig-backend-gconf 0.9.5.92-0ubuntu2 Compiz Fusion configuration system - gconf backend
ii compizconfig-settings-manager 0.9.5.92-0ubuntu3 Compiz configuration settings manager
ii libcompizconfig0 0.9.7.0~bzr428-0ubuntu3 Settings library for plugins - OpenCompositing Project
ii python-compizconfig 0.9.5.94-0ubuntu4 Compizconfig bindings for python
ii gnome-session-common 3.2.1-0ubuntu6 GNOME Session Manager - common files
ii gnome-session-fallback 3.2.1-0ubuntu6 GNOME Session Manager - GNOME fallback session
Attached is a video demonstrating the problem.