Blue resize/move background incorrectly positioned in 2D-space

[Shevek]

OK, this is a weird one.

When I drag or resize a window in xfwm, it draws a blue semi-transparent box, and uses that as the placeholder for the window I'm resizing or moving.

It's behaving as if the blue box is always positioned at logical (0, 0) instead of being positioned at the place the window was.

By which I mean, if I drag a window which was physically at (0, 0), I get a nice blue drag-box.

If I drag a window which was at (say) (100, 100) then the top left "most" of the window is represented by a blue drag-box, but the bottom and right 100px of the window are not represented in the drag. The blue box is clipped to the actual window position, but behaves as if it was always starting from (0, 0)

Resize is similar.

I can't screengrab it because I'm holding a mouse button down at the time... xwd can't grab correctly from this server, and ...

Notes: This is with the modesetting X server.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: xfwm4 4.12.1-1ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-9.9-generic 3.19.1
Uname: Linux 3.19.0-9-generic x86_64
ApportVersion: 2.16.2-0ubuntu4
Architecture: amd64
CurrentDesktop: XFCE
Date: Sun Mar 22 14:05:29 2015
InstallationDate: Installed on 2015-03-05 (17 days ago)
InstallationMedia: Xubuntu 14.04.2 LTS "Trusty Tahr" - Release amd64 (20150218.1)
SourcePackage: xfwm4
UpgradeStatus: Upgraded to vivid on 2015-03-19 (3 days ago)

[Shevek (r-launchpad-anarres-org)]

Resizing, the translucent blue box resizes correctly, but it's apparently rooted at (0, 0) not (window-x, window-y)

[Shevek (r-launchpad-anarres-org)]

Moving, the translucent blue box starts at (0, 0) and any bit of it which was overlapping with the window's original position is clipped to the window and moves with it.

[Shevek (r-launchpad-anarres-org)]

I'm told that this might be a bug in the modesetting driver's glamor acceleration module. Can't work out how to add xserver-xorg-core as an 'affects'.

[Thaddaeus Tintenfisch (thad-fisch-deactivatedaccount)]

Who told you that? Did you forward this report to the Xfce bug tracker?

[Shevek (r-launchpad-anarres-org)]

A friend and core X developer whose name I won't abuse told me.

No, I did not forward it to xfce. It's only reported here.

[Thaddaeus Tintenfisch (thad-fisch-deactivatedaccount)]

Can you please forward it to the upstream bug tracker? Thanks in advance.

[Launchpad Janitor (janitor)]

[Expired for xorg-server (Ubuntu) because there has been no activity for 60 days.]

[Launchpad Janitor (janitor)]

[Expired for xfwm4 (Ubuntu) because there has been no activity for 60 days.]

[Shevek (r-launchpad-anarres-org)]

It's still broken.