I'm not sure of the chain of events, but I am of the hang, since I got these stack traces for two of the threads. It was running with a private branch with the smartcard channel at the time (but I think it is not related).

#0  0x0000003e1760b8c4 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x0000003e19a4daf7 in _XReply (dpy=0xd26be0, rep=0x7fffb24a1e10, extra=0, discard=1) at xcb_io.c:526
#2  0x0000003e19a2b09b in _XGetWindowAttributes (dpy=0xd26be0, w=<value optimized out>, attr=0x7fffb24a1ee0) at GetWAttrs.c:115
#3  0x0000003e19a2b241 in XGetWindowAttributes (dpy=0xd26be0, w=<value optimized out>, attr=<value optimized out>) at GetWAttrs.c:150
#4  0x0000000000425791 in RedWindow::get_size (this=0xd4f2f0) at red_window.cpp:1751
#5  0x000000000042278c in RedWindow_p::win_proc (event=...) at red_window.cpp:782
#6  0x000000000040ddfa in XEventHandler::on_event (this=0xd4d4d0) at platform.cpp:285
#7  0x000000000040beef in EventSource::action (this=0xd4d4d0) at ../event_sources.h:56
#8  0x000000000040aa46 in EventSources::wait_events (this=0xd7adb8, timeout_ms=0) at event_sources_p.cpp:117
#9  0x000000000049f14a in ProcessLoop::run (this=0xd7adb0) at ../../client/process_loop.cpp:281
#10 0x000000000042be98 in Application::run (this=0xd7adb0) at ../../client/application.cpp:602
#11 0x0000000000432499 in Application::main (argc=5, argv=0x7fffb24a24c8, version_str=0x546020 "0.6.1") at ../../client/application.cpp:2324
#12 0x000000000040c7d5 in main (argc=5, argv=0x7fffb24a24c8) at main.cpp:34

and cursor channel thread:

#0  0x0000003e1760b8c4 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x0000003e19608b8f in _xcb_conn_wait (c=0xd27f40, cond=<value optimized out>, vector=0x0, count=0x0) at xcb_conn.c:274
#2  0x0000003e19609c8f in xcb_wait_for_reply (c=0xd27f40, request=4446, e=0x7f6b055c7558) at xcb_in.c:390
#3  0x0000003e19a4db3d in _XReply (dpy=0xd26be0, rep=0x7f6b055c75a0, extra=0, discard=1) at xcb_io.c:532
#4  0x0000003e19a414b3 in XSync (dpy=0xd26be0, discard=0) at Sync.c:46
#5  0x000000000040e03f in XPlatform::create_x_shm_image (format=RedDrawable::ARGB32, width=24, height=24, depth=24, visual=0xd31980, shminfo_out=0x7f6b055c7740) at platform.cpp:367
#6  0x000000000040e166 in XPlatform::create_x_image (format=RedDrawable::ARGB32, width=24, height=24, depth=24, visual=0xd31980, shminfo_out=0x7f6b055c7740) at platform.cpp:412
#7  0x0000000000421698 in RedPixmapSw::RedPixmapSw (this=0x7f6af00042b0, width=24, height=24, format=RedDrawable::ARGB32, top_bottom=true, win=0x0) at red_pixmap_sw.cpp:55
#8  0x000000000046c7c1 in AlphaCursor::AlphaCursor (this=0x7f6af00043b0, header=..., data=0x7f6af0002b90 "") at ../../client/cursor_channel.cpp:115
#9  0x000000000046da8c in CursorChannel::create_native_cursor (cursor=0x7f6af00009a0) at ../../client/cursor_channel.cpp:462
#10 0x000000000046de49 in CursorChannel::set_cursor (this=0x7f6b00007730, red_cursor=..., x=512, y=384, visible=true) at ../../client/cursor_channel.cpp:508
#11 0x000000000046e280 in CursorChannel::handle_cursor_set (this=0x7f6b00007730, message=0x7f6b055c7940) at ../../client/cursor_channel.cpp:578
#12 0x0000000000470438 in MessageHandlerImp<CursorChannel, 4u>::handle_message (this=0x7f6b00009c70, message=...) at ../../client/red_channel.h:318
#13 0x00000000004b7b6b in RedChannel::recive_messages (this=0x7f6b00007730) at ../../client/red_channel.cpp:603
#14 0x00000000004b7f03 in RedChannel::on_event (this=0x7f6b00007730) at ../../client/red_channel.cpp:650
#15 0x000000000040beef in EventSource::action (this=0x7f6b00007730) at ../event_sources.h:56
#16 0x000000000040aa46 in EventSources::wait_events (this=0x7f6b00007998, timeout_ms=-1) at event_sources_p.cpp:117
#17 0x000000000049f14a in ProcessLoop::run (this=0x7f6b00007990) at ../../client/process_loop.cpp:281
#18 0x00000000004b727a in RedChannel::run (this=0x7f6b00007730) at ../../client/red_channel.cpp:471
#19 0x00000000004b6900 in RedChannel::worker_main (data=0x7f6b00007730) at ../../client/red_channel.cpp:320
#20 0x0000003e17606d5b in start_thread () from /lib64/libpthread.so.0
#21 0x0000003e16ae3f6d in clone () from /lib64/libc.so.6