It's such a drag when you can't drag!

Strange problem that's been bugging me for awhile now:

For no apparent reason, the (Mac OS X) Finder suddenly chooses not to respond to an attempt to drag a file - say, to move it into another folder.

No error message; just that dislocated feeling when you realize that you didn't really pick it up - and can't.

Nothing interesting in syslog (asl) / system.log (even temporarily logging *.debug messages). Nothing in Console. Nothing in any other log that had recently been modified.

Tools like fs_usage and sc_usage didn't yield any promising clues.

Quitting and restarting the Finder didn't help.

HUP'ing SystemUIServer didn't help.

Checking the SystemUIServer plist ("defaults read /Users/username/Library/Preferences/com.apple.systemuiserver.plist") showed nothing unusual - and I don't just delete prefs to see what happens; that's sledgehammer voodoo.

Google searches turned up lots of discussion - but really nothing more interesting than the standard "Repair Permissions" voodoo or "Have you tried turning it off and on again?"

Quitting all open apps (and menu extras, etc.) had no effect.

Restarting will clear the problem (as will, if I recall, logging out and back in) - for awhile - however of course it's then necessary to reconstruct the 17 things you were trying to do at the moment; not good.

I noticed that other apps were affected - for instance, TextWrangler reports an error -4960 when attempting a drag.

The very cool "Technical Note TN2124 - Mac OS X Debugging Magic" (on the Apple Developer site) had a promising idea (launch an app with "-NSDragManagerLogLevel 6"), though that didn't yield anything for either Finder (probably because it's not a Cocoa app :) or TextWrangler (is it a Cocoa app?).

Looking into the Drag Manager some more didn't yield anything promising.

Well, then I started searching more deeply, including into some of the coding discussions. That's when I found mentions of the pasteboard (AKA clipboard). It didn't seem related, however that was the trick:

sudo killall -s pboard

(HUP'ing it apparently is not sufficient, but the default signal (-TERM) will do it. The "-s" flag means it'll show you what it'll do; if you approve, re-issue the command without the "-s" to really do it.)

It seems that - even for dragging a file in the Finder - the pasteboard is engaged and if it's wedged the operation fails silently.

If I can discover a little more background, I'll submit to RADAR.)

UPDATE: Apparently a little more background is required: While killing pboard re-allows dragging, it apparently is a BAD thing, causing apps to hang, probably expecting to hear back from the old process. Back to the drawing board...

Update 2009/03/20: It happened again and while I confirmed that logging out will clear it, that's not good enough. I did however find that there was a "Recovered Items" folder in my Trash; it contained a folder called "msoclip1" with four files (clip_image001.gif, clip_image002.wmf, clip_image003.png, clip_oledata.mso) so maybe the problem is related to some MS Office app's use of the clipboard?