Network interfaces die when laptop lid closes in Fluxbox

[Brian Cottingham <spiffytech@gmail.com>]

I've installed Debian Jessie on a new computer. When I run Fluxbox or
XFCE and close my laptop lid (the laptop does not sleep when the lid
closes), all network interfaces are disconnected and NetworkManager
doesn't know they exist anymore ("Networking is disabled". Clicking
"Enable networking" has no effect.). I have to restart the networking
and network-manager services for them to work again.

This doesn't happen if I run KDE.

Here is my output from `tail -F /var/log/syslog` when I close and
reopen the laptop lid: http://pastebin.com/gcVSqeg7

I suspect this is a NetworkManager problem, since if I stop the
network-manager service and configure the network interfaces manually,
they don't disconnect. I do see NetworkManager doing some stuff in the
log that sound an awful lot like disabling my network, but I can't
figure out why it would do that.

What could be going on here?