After upgrading to kernel 5.12.7, sometimes X would crash when I made windows fullscreen. I downgraded my kernel to 5.11.22 and haven't had any crashes with that kernel.

Here's a stacktrace of a crash:

Stack trace of thread 36097:
#0 0x0000624acea71d22 raise (libc.so.6 + 0x3cd22)
#1 0x0000624acea5b862 abort (libc.so.6 + 0x26862)
#2 0x00000bff2f1ac75a OsAbort (Xorg + 0x14a75a)
#3 0x00000bff2f1ae221 FatalError (Xorg + 0x14c221)
#4 0x00000bff2f1b3e59 n/a (Xorg + 0x151e59)
#5 0x0000624acea71da0 __restore_rt (libc.so.6 + 0x3cda0)
#6 0x00000bff2f123fde n/a (Xorg + 0xc1fde)
#7 0x00000bff2f21b611 n/a (Xorg + 0x1b9611)
#8 0x00000bff2f134a72 n/a (Xorg + 0xd2a72)
#9 0x00000bff2f13b241 n/a (Xorg + 0xd9241)
#10 0x00000bff2f13cbac n/a (Xorg + 0xdabac)
#11 0x00000bff2f1392ec n/a (Xorg + 0xd72ec)
#12 0x00000bff2f09c195 n/a (Xorg + 0x3a195)
#13 0x0000624acea5cb25 __libc_start_main (libc.so.6 + 0x27b25)
#14 0x00000bff2f09c5de _start (Xorg + 0x3a5de)

Stack trace of thread 36098:
#0 0x0000624ace92f8ca __futex_abstimed_wait_common64 (libpthread.so.0 + 0x158ca)
#1 0x0000624ace929270 pthread_cond_wait@@GLIBC_2.3.2 (libpthread.so.0 + 0xf270)
#2 0x0000624acdc75b90 n/a (intel_drv.so + 0xb0b90)
#3 0x0000624ace923259 start_thread (libpthread.so.0 + 0x9259)
#4 0x0000624aceb335e3 __clone (libc.so.6 + 0xfe5e3)

Stack trace of thread 36103:
#0 0x0000624ace92f8ca __futex_abstimed_wait_common64 (libpthread.so.0 + 0x158ca)
#1 0x0000624ace929270 pthread_cond_wait@@GLIBC_2.3.2 (libpthread.so.0 + 0xf270)
#2 0x0000624acc44f15c n/a (i965_dri.so + 0x24015c)
#3 0x0000624acc44eb18 n/a (i965_dri.so + 0x23fb18)
#4 0x0000624ace923259 start_thread (libpthread.so.0 + 0x9259)
#5 0x0000624aceb335e3 __clone (libc.so.6 + 0xfe5e3)

Stack trace of thread 36100:
#0 0x0000624ace92f8ca __futex_abstimed_wait_common64 (libpthread.so.0 + 0x158ca)
#1 0x0000624ace929270 pthread_cond_wait@@GLIBC_2.3.2 (libpthread.so.0 + 0xf270)
#2 0x0000624acc44f15c n/a (i965_dri.so + 0x24015c)
#3 0x0000624acc44eb18 n/a (i965_dri.so + 0x23fb18)
#4 0x0000624ace923259 start_thread (libpthread.so.0 + 0x9259)
#5 0x0000624aceb335e3 __clone (libc.so.6 + 0xfe5e3)

Stack trace of thread 36102:
#0 0x0000624ace92f8ca __futex_abstimed_wait_common64 (libpthread.so.0 + 0x158ca)
#1 0x0000624ace929270 pthread_cond_wait@@GLIBC_2.3.2 (libpthread.so.0 + 0xf270)
#2 0x0000624acc44f15c n/a (i965_dri.so + 0x24015c)
#3 0x0000624acc44eb18 n/a (i965_dri.so + 0x23fb18)
#4 0x0000624ace923259 start_thread (libpthread.so.0 + 0x9259)
#5 0x0000624aceb335e3 __clone (libc.so.6 + 0xfe5e3)

Stack trace of thread 36101:
#0 0x0000624ace92f8ca __futex_abstimed_wait_common64 (libpthread.so.0 + 0x158ca)
#1 0x0000624ace929270 pthread_cond_wait@@GLIBC_2.3.2 (libpthread.so.0 + 0xf270)
#2 0x0000624acc44f15c n/a (i965_dri.so + 0x24015c)
#3 0x0000624acc44eb18 n/a (i965_dri.so + 0x23fb18)
#4 0x0000624ace923259 start_thread (libpthread.so.0 + 0x9259)
#5 0x0000624aceb335e3 __clone (libc.so.6 + 0xfe5e3)