This may not be Unity8 specific. I got a similar lockup in miral-shell.

1. Use mir_demo_shell as a host:
   $ sudo mir_demo_server --arw-file --vt 4 --window-manager system-compositor

2. Run miral-shell as a guest:
   $ miral-app --host /tmp/mir_socket

3. Start egltriangle & eglplasma and start switching, resizing, maximizing etc. After a while it seems the "RPC thread" starts making blocking calls to the client API.

(gdb) info threads
  Id   Target Id         Frame 
* 1    Thread 0x7f1811eafc80 (LWP 4525) "miral-shell" 0x00007f181036e18d in poll () at ../sysdeps/unix/syscall-template.S:84
  2    Thread 0x7f180a305700 (LWP 4526) "RPC Thread" pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  3    Thread 0x7f1803fff700 (LWP 4532) "Mir/Snapshot" pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  4    Thread 0x7f18037fe700 (LWP 4533) "Mir/Comp" pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  5    Thread 0x7f1802ffd700 (LWP 4534) "Mir/Input Reade" 0x00007f181036e18d in poll () at ../sysdeps/unix/syscall-template.S:84
  6    Thread 0x7f18027fc700 (LWP 4535) "Mir/IPC" pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  7    Thread 0x7f1801ffb700 (LWP 4536) "Mir/IPC" pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  8    Thread 0x7f18017fa700 (LWP 4537) "RPC Thread" 0x00007f181036e18d in poll () at ../sysdeps/unix/syscall-template.S:84
  9    Thread 0x7f1800ff9700 (LWP 4538) "miral-shell" pthread_cond_wait@@GLIBC_2.3.2 ()
    at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
(gdb) t 2
[Switching to thread 2 (Thread 0x7f180a305700 (LWP 4526))]
#0  pthread_cond_wait@@GLIBC_2.3.2 () at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
185	../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S: No such file or directory.
(gdb) bt
#0  0x00007f180f842510 in pthread_cond_wait@@GLIBC_2.3.2 () at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
#1  0x00007f181090580c in std::condition_variable::wait(std::unique_lock<std::mutex>&) () at /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#2  0x00007f1810e3c18b in wait () at /usr/include/c++/6/condition_variable:99
#3  0x00007f1810e3c18b in wait_for_result (this=0x7f180a303e00) at ./src/client/mir_render_surface_api.cpp:43
#4  0x00007f1810e3c18b in mir_connection_create_render_surface_sync (connection=<optimised out>, width=<optimised out>, height=<optimised out>) at ./src/client/mir_render_surface_api.cpp:126
#5  0x00007f180ffc4f67 in set_cursor_image (image=..., this=0x55c1dbfb8540)
    at ./src/server/graphics/nested/mir_client_host_connection.cpp:177
#6  0x00007f180ffc4f67 in set_cursor_image (this=0x55c1dbd70940, image=...)
    at ./src/server/graphics/nested/mir_client_host_connection.cpp:491
#7  0x00007f180ffcc076 in mir::input::CursorController::update_cursor_image_locked(std::unique_lock<std::mutex>&) [clone .constprop.589] (this=this@entry=0x55c1dbe82f10, lock=...) at ./src/server/input/cursor_controller.cpp:248
#8  0x00007f180ff2487c in update_cursor_image (this=0x55c1dbe82f10) at ./src/server/input/cursor_controller.cpp:259
#9  0x00007f180ff3c1c3 in std::function<void (std::shared_ptr<mir::scene::SurfaceObserver> const&)>::operator()(std::shared_ptr<mir::scene::SurfaceObserver> const&) const () at /usr/include/c++/6/functional:2127
#10 0x00007f180ff3c1c3 in for_each (this=<optimised out>, f=...) at ./src/include/common/mir/thread_safe_list.h:81
#11 0x00007f180ff3cf11 in mir::scene::SurfaceObservers::moved_to(mir::geometry::Point const&) (this=<optimised out>, top_left=...)
    at ./src/server/scene/basic_surface.cpp:65
#12 0x00007f1811aaf378 in miral::Window::move_to(mir::geometry::Point) (this=<optimised out>, top_left=...) at ./miral/window.cpp:74
#13 0x00007f1811abe071 in miral::BasicWindowManager::move_tree(miral::WindowInfo&, mir::geometry::Displacement) (this=this@entry=0x55c1dbfdf540, root=..., movement=...) at ./miral/basic_window_manager.cpp:807
#14 0x00007f1811abb507 in miral::BasicWindowManager::place_and_size(miral::WindowInfo&, mir::geometry::Point const&, mir::geometry::Size const&) (this=this@entry=0x55c1dbfdf540, root=..., new_pos=..., new_size=...) at ./miral/basic_window_manager.cpp:999
#15 0x00007f1811abe831 in miral::BasicWindowManager::modify_window(miral::WindowInfo&, miral::WindowSpecification const&) (this=0x55c1dbfdf540, window_info=..., modifications=...) at ./miral/basic_window_manager.cpp:933