Hi all,

Here's another backtrace, though It's still unclear to me what caused it, and I can't reproduce it reliably:

> (gdb) run
> Starting program: /build/emacs/src/emacs 
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
> [New Thread 0x7fffe9d0d700 (LWP 5382)]
> [New Thread 0x7fffe3fff700 (LWP 5383)]
> [New Thread 0x7fffe37fe700 (LWP 5384)]
> 
> xdisp.c:27395: Emacs fatal error: assertion failed: it->ascent >= 0 && it->descent >= 0
> Fatal error 6: Aborted
> Backtrace:
> /build/emacs/src/emacs[0x5b53a8]
> /build/emacs/src/emacs[0x588afe]
> /build/emacs/src/emacs[0x61a40d]
> /build/emacs/src/emacs[0x4a24f7]
> /build/emacs/src/emacs[0x4865db]
> /build/emacs/src/emacs[0x47861d]
> /build/emacs/src/emacs[0x474b33]
> /build/emacs/src/emacs[0x46b77e]
> /build/emacs/src/emacs[0x63bb67]
> /build/emacs/src/emacs[0x46b6e2]
> /build/emacs/src/emacs[0x46b698]
> /build/emacs/src/emacs[0x46a391]
> /build/emacs/src/emacs[0x467b38]
> /build/emacs/src/emacs[0x590ca5]
> /build/emacs/src/emacs[0x5a1963]
> /build/emacs/src/emacs[0x58d96c]
> /build/emacs/src/emacs[0x63bacd]
> /build/emacs/src/emacs[0x58d14b]
> /build/emacs/src/emacs[0x63b09e]
> /build/emacs/src/emacs[0x58d0eb]
> /build/emacs/src/emacs[0x58c64a]
> /build/emacs/src/emacs[0x58c84f]
> /build/emacs/src/emacs[0x58a559]
> /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5)[0x7ffff0f55ec5]
> /build/emacs/src/emacs[0x417f99]
> 
> Program received signal SIGABRT, Aborted.
> 0x00007ffff130920b in raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/pt-raise.c:37
> 37	../nptl/sysdeps/unix/sysv/linux/pt-raise.c: No such file or directory.
> (gdb) backtrace
> #0  0x00007ffff130920b in raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/pt-raise.c:37
> #1  0x0000000000588b41 in terminate_due_to_signal (sig=6, backtrace_limit=2147483647) at emacs.c:398
> #2  0x000000000061a40d in die (msg=0x712f40 "it->ascent >= 0 && it->descent >= 0", file=0x70e890 "xdisp.c", line=27395) at alloc.c:7036
> #3  0x00000000004a24f7 in x_produce_glyphs (it=0x7fffffff8530) at xdisp.c:27395
> #4  0x00000000004865db in display_line (it=0x7fffffff8530) at xdisp.c:20491
> #5  0x000000000047861d in try_window (window=51499077, pos=..., flags=1) at xdisp.c:17089
> #6  0x0000000000474b33 in redisplay_window (window=51499077, just_this_one_p=false) at xdisp.c:16554
> #7  0x000000000046b77e in redisplay_window_0 (window=51499077) at xdisp.c:14346
> #8  0x000000000063bb67 in internal_condition_case_1 (bfun=0x46b73c <redisplay_window_0>, arg=51499077, handlers=14472099, hfun=0x46b704 <redisplay_window_error>) at eval.c:1333
> #9  0x000000000046b6e2 in redisplay_windows (window=51499077) at xdisp.c:14326
> #10 0x000000000046b698 in redisplay_windows (window=50476549) at xdisp.c:14320
> #11 0x000000000046a391 in redisplay_internal () at xdisp.c:13886
> #12 0x0000000000467b38 in redisplay () at xdisp.c:13119
> #13 0x0000000000590ca5 in read_char (commandflag=1, map=64460675, prev_event=0, used_mouse_menu=0x7fffffffd759, end_time=0x0) at keyboard.c:2463
> #14 0x00000000005a1963 in read_key_sequence (keybuf=0x7fffffffd930, bufsize=30, prompt=0, dont_downcase_last=false, can_return_switch_frame=true, fix_current_buffer=true, 
>     prevent_redisplay=false) at keyboard.c:9024
> #15 0x000000000058d96c in command_loop_1 () at keyboard.c:1344
> #16 0x000000000063bacd in internal_condition_case (bfun=0x58d51d <command_loop_1>, handlers=18960, hfun=0x58cb65 <cmd_error>) at eval.c:1309
> #17 0x000000000058d14b in command_loop_2 (ignore=0) at keyboard.c:1087
> #18 0x000000000063b09e in internal_catch (tag=45744, func=0x58d122 <command_loop_2>, arg=0) at eval.c:1074
> #19 0x000000000058d0eb in command_loop () at keyboard.c:1066
> #20 0x000000000058c64a in recursive_edit_1 () at keyboard.c:672
> #21 0x000000000058c84f in Frecursive_edit () at keyboard.c:743
> #22 0x000000000058a559 in main (argc=1, argv=0x7fffffffdd58) at emacs.c:1656
> (gdb) 

Again, let me know if there's anything I can do to help with this.
Clément.