Middle mouse button behavior changed?

I'm running Debian testing/stretch on a Thinkpad T520i.

I'm using the internal trackpoint and mouse buttons, I've disabled the trackpad (in BIOS).

Following a recent update and reboot, my middle mouse behavior has changed.

I can still use it to copy and past through the Xorg clipboard.

Testing:

I can still use it to copy and past through the Xorg clipboard.

Yep, that worked.

But in other contexts, it no longer seems to work via applications.

In Windowmaker, it no longer invokes the Windows menu (my default binding for that).
If I hold the middle button, I cannot move the mouse pointer. This applies both in Xorg and on a console with gpm. Which strongly suggests the problem isn't an Xorg, window manager, or application specific issue.
In the mission-critical application freeciv, middle mouse no longer brings up tile information. I'll confess I noted the problem here first.

I'm somewhat suspicious it's the kernel. From uname:

3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt4-3 (2015-02-03) x86_64 GNU/Linux

From dmesg:

[    0.586928] mousedev: PS/2 mouse device common for all mice
[    1.262800] psmouse serio1: alps: Unknown ALPS touchpad: E7=10 00 64, EC=10 00 64
[    1.658713] psmouse serio1: trackpoint: IBM TrackPoint firmware: 0x0e, buttons: 3/3

xev (from x11-utils

reports, on pressing buttons one, two, and three respectively:

ButtonPress event, serial 36, synthetic NO, window 0x3c00001,
    root 0xc4, subw 0x0, time 5790137, (137,71), root:(874,738),
    state 0x0, button 1, same_screen YES

ButtonRelease event, serial 36, synthetic NO, window 0x3c00001,
    root 0xc4, subw 0x0, time 5790295, (137,71), root:(874,738),
    state 0x100, button 1, same_screen YES

ButtonPress event, serial 36, synthetic NO, window 0x3c00001,
    root 0xc4, subw 0x0, time 5791324, (137,71), root:(874,738),
    state 0x0, button 2, same_screen YES

ButtonRelease event, serial 36, synthetic NO, window 0x3c00001,
    root 0xc4, subw 0x0, time 5791324, (137,71), root:(874,738),
    state 0x200, button 2, same_screen YES

ButtonPress event, serial 36, synthetic NO, window 0x3c00001,
    root 0xc4, subw 0x0, time 5791729, (137,71), root:(874,738),
    state 0x0, button 3, same_screen YES

ButtonRelease event, serial 36, synthetic NO, window 0x3c00001,
    root 0xc4, subw 0x0, time 5791878, (137,71), root:(874,738),
    state 0x400, button 3, same_screen YES

Suggestions or pointers (which work) welcomed.

(Also posted to /r/linux on Reddit: https://redd.it/4on2f8)

Dr. Ed Morbius
Chief Scientist / Philologist / Robot Wrangler / Powerplant Operator
Krell Power Systems Unlimited

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