After testing xfce4 4.14 on Xubuntu 19.04, I installed the staging PPA from https://launchpad.net/~xubuntu-dev/+archive/ubuntu/staging in an 18.04 install. It was noticed that xfwm4 crashes using the i965 driver from either Mesa 19.0.2 from default Ubuntu install and Mesa git. For Mesa git, there is the workaround to use the Iris driver, which has many graphical bugs, but at least works.

The general workaround is to use xpresent instead of auto or glx.

The crash has the following backtrace:

#0  0x00007ffff3688e97 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007ffff368a801 in __GI_abort () at abort.c:79
#2  0x00007ffff367a39a in __assert_fail_base (fmt=0x7ffff7fc3202 <error: Não é possível acessar a memória no endereço 0x7ffff7fc3202>, assertion=assertion@entry=0x7fffe8da3b08 "image->TexObject->Target == mt->target", file=file@entry=0x7fffe8da39d8 "../src/mesa/drivers/dri/i965/intel_mipmap_tree.c", line=line@entry=1285, function=function@entry=0x7fffe8da4ed0 <__PRETTY_FUNCTION__.38427> "intel_miptree_match_image") at assert.c:92
#3  0x00007ffff367a412 in __GI___assert_fail (assertion=assertion@entry=0x7fffe8da3b08 "image->TexObject->Target == mt->target", file=file@entry=0x7fffe8da39d8 "../src/mesa/drivers/dri/i965/intel_mipmap_tree.c", line=line@entry=1285, function=function@entry=0x7fffe8da4ed0 <__PRETTY_FUNCTION__.38427> "intel_miptree_match_image") at assert.c:101
#4  0x00007fffe8720f43 in intel_miptree_match_image (mt=<optimized out>, image=image@entry=0x555555f51300) at ../src/mesa/drivers/dri/i965/intel_mipmap_tree.c:1285
#5  0x00007fffe87303c9 in intel_finalize_mipmap_tree (brw=brw@entry=0x555555d02a50, tObj=0x555555eed4b0) at ../src/mesa/drivers/dri/i965/intel_tex_validate.c:112
#6  0x00007fffe87306e4 in brw_validate_textures (brw=brw@entry=0x555555d02a50) at ../src/mesa/drivers/dri/i965/intel_tex_validate.c:195
#7  0x00007fffe86f3574 in brw_prepare_drawing (max_index=3, min_index=0, index_bounds_valid=<optimized out>, ib=0x0, ctx=0x555555d02a50) at ../src/mesa/drivers/dri/i965/brw_draw.c:811
#8  0x00007fffe86f3574 in brw_draw_prims (ctx=0x555555d02a50, prims=0x555555d390a8, nr_prims=1, ib=0x0, index_bounds_valid=<optimized out>, min_index=<optimized out>, max_index=<optimized out>, gl_xfb_obj=0x0, stream=0, indirect=0x0) at ../src/mesa/drivers/dri/i965/brw_draw.c:1133
#9  0x00007fffe889cde4 in vbo_exec_vtx_flush (exec=exec@entry=0x555555d388b0, keepUnmapped=<optimized out>) at ../src/mesa/vbo/vbo_exec_draw.c:388
#10 0x00007fffe887fb54 in vbo_exec_FlushVertices_internal (exec=0x555555d388b0, unmap=<optimized out>) at ../src/mesa/vbo/vbo_exec_api.c:638
#11 0x00007fffe889c6c3 in vbo_exec_FlushVertices (ctx=ctx@entry=0x555555d02a50, flags=flags@entry=1) at ../src/mesa/vbo/vbo_exec_api.c:1256
#12 0x00007fffe872af6d in intel_dri2_flush_with_flags (cPriv=<optimized out>, dPriv=0x5555559765e0, flags=3, reason=__DRI2_THROTTLE_SWAPBUFFER) at ../src/mesa/drivers/dri/i965/intel_screen.c:150
#13 0x00007fffea5a86d3 in loader_dri3_swap_buffers_msc (draw=0x555555d3cec8, target_msc=0, divisor=0, remainder=0, flush_flags=<optimized out>, force_copy=<optimized out>) at ../src/loader/loader_dri3_helper.c:917
#14 0x000055555556f2ec in  ()
#15 0x00007ffff4b98d03 in g_timeout_dispatch (source=source@entry=0x555555f85f70, callback=<optimized out>, user_data=<optimized out>) at ../../../../glib/gmain.c:4649
#16 0x00007ffff4b98285 in g_main_dispatch (context=0x5555557fe3f0) at ../../../../glib/gmain.c:3176
#17 0x00007ffff4b98285 in g_main_context_dispatch (context=context@entry=0x5555557fe3f0) at ../../../../glib/gmain.c:3829
#18 0x00007ffff4b98650 in g_main_context_iterate (context=0x5555557fe3f0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../../../../glib/gmain.c:3902
#19 0x00007ffff4b98962 in g_main_loop_run (loop=0x555555f059e0) at ../../../../glib/gmain.c:4098
#20 0x00007ffff63a2a25 in gtk_main () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#21 0x0000555555564191 in  ()
#22 0x00007ffff366bb97 in __libc_start_main (main=0x555555563c10, argc=3, argv=0x7fffffffe148, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffe138) at ../csu/libc-start.c:310
#23 0x000055555556454a in  ()

xfwm4 --version:

	This is xfwm4 version 4.13.4 (revision b1872fda) for Xfce 4.14pre2
	Released under the terms of the GNU General Public License.
	Compiled against GTK+-3.22.30, using GTK+-3.22.30.

	Build configuration and supported features:
	- Startup notification support:                 Yes
	- XSync support:                                Yes
	- Render support:                               Yes
	- Xrandr support:                               Yes
	- Xpresent support:                             Yes
	- Embedded compositor:                          Yes
	- Epoxy support:                                Yes
	- KDE systray proxy (deprecated):               No


System specifications:

Processor: Intel Core i3-6100U;
Video: Intel HD Graphics 520;
Architecture: amd64;
RAM memory: 20 GB;
Mesa: 19.0.2;
Kernel version: 4.19.60-041960-lowlatency;
Distribution: Xubuntu 18.04.2 amd64.

Should I report this bug against Mesa too, as many parts of the backtrace are related to i965? Anyways, due to the apparent frequent bugs caused by the glx vblank_mode (I had problems in 2/3 of my installs), isn't valid to consider reverting to xpresent as default for now?