bug#52947: 29.0.50; Newline in tab-bar-format causes lockup

[Juri Linkov]

> Do we want to support newlines there? what would be the purpose of
> having newlines in the tabs?

I don't know, menus can't have newlines too.

> Also, you say "display engine", so does it mean you've found where we
> infloop with these recipes?  If so, can you tell the details?

It gives such backtrace:

(gdb) bt
#0  make_lisp_symbol (sym=0x555555cfd9c0 <lispsym>) at lisp.h:1002
#1  0x00005555555b9766 in builtin_lisp_symbol (index=0) at lisp.h:1008
#2  0x00005555555b9cf6 in CHAR_TABLE_REF_ASCII (ct=XIL(0x7ffff0b438dd), idx=10) 
at lisp.h:2031
#3  0x00005555555b9d3e in CHAR_TABLE_REF (ct=XIL(0x7ffff0b438dd), idx=10) at 
lisp.h:2042
#4  0x00005555555d16de in lookup_glyphless_char_display (c=10, 
it=0x7fffffff75d0) at xdisp.c:7410
#5  0x00005555555d1f8c in get_next_display_element (it=0x7fffffff75d0) at 
xdisp.c:7599
#6  0x00005555555e3133 in display_tab_bar_line (it=0x7fffffff75d0, height=-1) 
at xdisp.c:13389
#7  0x00005555555e3901 in tab_bar_height (f=0x5555560a2dc8, 
n_rows=0x5555560a2f08, pixelwise=true) at xdisp.c:13515
#8  0x00005555555e3c27 in redisplay_tab_bar (f=0x5555560a2dc8) at xdisp.c:13597