With an intel GPU, Mir trunk, latest wily,  moving the client window between screens causes the nested server to crash soon after (although not immediately). The backtrace I get is:

#0  0x00007ffff7089267 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:55
#1  0x00007ffff708aeca in __GI_abort () at abort.c:89
#2  0x00007ffff70ccc53 in __libc_message (do_abort=do_abort@entry=1, 
    fmt=fmt@entry=0x7ffff71e51a8 "*** Error in `%s': %s: 0x%s ***\n")
    at ../sysdeps/posix/libc_fatal.c:175
#3  0x00007ffff70d4c69 in malloc_printerr (ptr=<optimized out>, 
    str=0x7ffff71e12fa "free(): invalid pointer", action=1) at malloc.c:4965
#4  _int_free (av=<optimized out>, p=<optimized out>, have_lock=0) at malloc.c:3834
#5  0x00007ffff70d889c in __GI___libc_free (mem=<optimized out>) at malloc.c:2950
#6  0x00007fffeba360b9 in _mesa_delete_program (ctx=<optimized out>, prog=0x76fde0)
    at ../../../../src/mesa/program/program.c:407
#7  0x00007fffeba3617b in _mesa_reference_program_ (ctx=ctx@entry=0x7ffff7fcf038, 
    ptr=ptr@entry=0x7b3870, prog=prog@entry=0x0) at ../../../../src/mesa/program/program.c:483
#8  0x00007fffeba18b37 in _mesa_reference_program (prog=0x0, ptr=<optimized out>, ctx=0x7ffff7fcf038)
    at ../../../../src/mesa/program/program.h:121
#9  _mesa_reference_vertprog (prog=0x0, ptr=<optimized out>, ctx=0x7ffff7fcf038)
    at ../../../../src/mesa/program/program.h:129
#10 _mesa_meta_end (ctx=ctx@entry=0x7ffff7fcf038) at ../../../../src/mesa/drivers/common/meta.c:961
#11 0x00007fffeba19448 in meta_clear (ctx=ctx@entry=0x7ffff7fcf038, buffers=buffers@entry=2, 
    glsl=glsl@entry=true) at ../../../../src/mesa/drivers/common/meta.c:1817
#12 0x00007fffeba1991a in _mesa_meta_glsl_Clear (ctx=ctx@entry=0x7ffff7fcf038, buffers=buffers@entry=2)
    at ../../../../src/mesa/drivers/common/meta.c:1502
#13 0x00007fffebb50e4b in brw_clear (ctx=0x7ffff7fcf038, mask=0)
    at ../../../../../../../src/mesa/drivers/dri/i965/brw_clear.c:264
#14 0x00007ffff687fe0f in mir::renderer::gl::Renderer::render (this=0x7fffd40008c0, 
    renderables=std::vector of length 2, capacity 2 = {...})
    at /storage/work/mir/src/renderers/gl/renderer.cpp:198
#15 0x00007ffff67eca32 in mir::compositor::DefaultDisplayBufferCompositor::composite(std::vector<std::shared_ptr<mir::compositor::SceneElement>, std::allocator<std::shared_ptr<mir::compositor::SceneElement> > >&&) (this=this@entry=0x7fffd404a250, 
    scene_elements=scene_elements@entry=<unknown type in /storage/work/mir/build/lib/libmirserver.so.35, CU 0x727f01, DIE 0x7349b4>)
    at /storage/work/mir/src/server/compositor/default_display_buffer_compositor.cpp:83
---Type <return> to continue, or q <return> to quit---
#16 0x00007ffff67f3f1c in mir::compositor::CompositingFunctor::operator() (this=0x821c70)
    at /storage/work/mir/src/server/compositor/multi_threaded_compositor.cpp:143
#17 0x00007ffff687c261 in std::function<void ()>::operator()() const (this=0x7fffe8b7ee70)
    at /usr/include/c++/5/functional:2271
#18 (anonymous namespace)::Task::execute (this=0x7fffe8b7ee70)
    at /storage/work/mir/src/server/thread/basic_thread_pool.cpp:40
#19 (anonymous namespace)::Worker::operator() (this=0x821d70)
    at /storage/work/mir/src/server/thread/basic_thread_pool.cpp:91
#20 std::__invoke<(anonymous namespace)::Worker> (__f=...) at /usr/include/c++/5/functional:201
#21 std::reference_wrapper<(anonymous namespace)::Worker>::operator()<> (this=<optimized out>)
    at /usr/include/c++/5/functional:428
#22 std::_Bind_simple<std::reference_wrapper<(anonymous namespace)::Worker>()>::_M_invoke<> (
    this=<optimized out>) at /usr/include/c++/5/functional:1531
#23 std::_Bind_simple<std::reference_wrapper<(anonymous namespace)::Worker>()>::operator() (
    this=<optimized out>) at /usr/include/c++/5/functional:1520
#24 std::thread::_Impl<std::_Bind_simple<std::reference_wrapper<(anonymous namespace)::Worker>()> >::_M_run(void) (this=<optimized out>) at /usr/include/c++/5/thread:115
#25 0x00007ffff790c030 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#26 0x00007ffff74256aa in start_thread (arg=0x7fffe8b7f700) at pthread_create.c:333
#27 0x00007ffff715aeed in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109