window snapping to the right edge is making the windows one or two pixels too narrow

Confirmed here too.

It renders essential functionality of the package (or a dependent one) broken.

I can't reproduce this on any of my up-to-date machines. Could you please attach a screenshot so we can get a good idea of what we're dealing with?

sure. attached an example.

This is Compiz issue and was introduced by this MP: https://code.launchpad.net/~3v1n0/compiz/extents-clamp-workarea/+merge/210202

That MP is protecting against bleeding the window's extents into another work area. Since the extents allow for mouse grabbing window resizing, there is a small gap allowing for the window to be resized.

I don't think that MP took into account semi-maximized windows. I'll take a look and see if we can discount the extent when semi-maximized.

This bug was fixed in the package compiz - 1:0.9.12.1+15.04.20150410.1-0ubuntu1

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compiz (1:0.9.12.1+15.04.20150410.1-0ubuntu1) vivid; urgency=medium

  [ CI Train Bot ]
  * New rebuild forced.

  [ Chris Townsend ]
  * Expo plugin: Make sure the Expo animation is completed before
    allowing clicks to be handled. (LP: #1026553)
  * Grid plugin: Float to int division truncating caused a 1 px gap on
    the right side of right semi-maximized window when the workspace has
    an odd width. Fix is to add 0.5 to the float division first to
    account for any truncation error. (LP: #1294864)

  [ Eleni Maria Stea ]
  * Change in GLVector implementation
  * Minor change: when the skydome was working (in the c version of
    compiz) it was rendered on top of the background so, there was no
    need to clear the screen before drawing (the skydome was rendered on
    top of the garbage from previous renderings, hiding it). In the c++
    plugin, the skydome is buggy and the background is visible (as well
    as some garbage from previous renderings). A temporary
    glClear(GL_COLOR_BUFFER_BIT) was added until we fix the skydome and
    the other cube bugs.
  * glDisableOutputClipping was not implemented in 3d.cpp => a parent
    class glDisableOutputClipping was called, causing 2d clipping
    artifacts (the 3d cube was clipped and you couldn't see the windows
    move)

  [ Marco Trevisan (Treviño) ]
  * OpenGL: don't hardcode X11 sync blacklisted models, but use settings
    instead (LP: #1441190)

  [ Stephen M. Webb ]
  * (Ubuntu packaging) migrate migration scripts to Python3. (LP:
    #1440548)
  * exclude Wine applications from window close animations (LP: #957879)
 -- CI Train Bot <email address hidden> Fri, 10 Apr 2015 21:55:34 +0000

This bug was fixed in the package unity - 7.3.2+15.04.20150410.1-0ubuntu1

---------------
unity (7.3.2+15.04.20150410.1-0ubuntu1) vivid; urgency=medium

  [ Brandon Schaefer ]
  * When we get a launcher key press (ie. super key). We check if the
    command lens is open. If it is then just switch the lens to the home
    lens. This way we dont close the dash. (LP: #1108956)

  [ CI Train Bot ]
  * New rebuild forced.

  [ Gerry Boland ]
  * SwitcherController: add first selection mode property, to decide
    what to select at first (LP: #998285)

  [ Marco Trevisan (Treviño) ]
  * SwitcherController: add first selection mode property, to decide
    what to select at first (LP: #998285)
  * UnityScreen: terminate spread if active when initiating Switcher
    (LP: #1412001)

  [ Stephen M. Webb ]
  * (Ubuntu packaging) move the session migration scripts to Python3
    (LP: #1440560)
  * fixed the Panel title string expected by Launcher keynav AP tests
    (LP: #1435830)
  * test_wm_keybindings: fix AP failures caused by upstream Compiz
    change (LP: #1294864)
 -- CI Train Bot <email address hidden> Fri, 10 Apr 2015 21:59:12 +0000