If the frame is split vertically with C-x 2 first, then Emacs doesn't
get stuck, but does the equivalent of (scroll-left 4) about once per
second; this can be interrupted by C-g.

Modifying the :align-to expression to `(+ ,(1- (window-hscroll)) (0
. right)) causes this assertion failure after some looping:

xdisp.c:19474: Emacs fatal error: assertion failed: row->pixel_width >= 0

Thread 1 "emacs" hit Breakpoint 1, terminate_due_to_signal (sig=6, backtrace_limit=2147483647)
    at emacs.c:354
354	  signal (sig, SIG_DFL);
(gdb) bt
#0  terminate_due_to_signal (sig=6, backtrace_limit=2147483647) at emacs.c:354
#1  0x00000000005fb0aa in die (msg=0x6f64c2 "row->pixel_width >= 0", file=0x6f3450 "xdisp.c", 
    line=19474) at alloc.c:7223
#2  0x0000000000479b64 in compute_line_metrics (it=0x7fffffff9110) at xdisp.c:19474
#3  0x0000000000481e07 in display_line (it=0x7fffffff9110) at xdisp.c:21234
#4  0x0000000000471b56 in try_window (window=21548085, pos=..., flags=1) at xdisp.c:17246
#5  0x000000000046e6ae in redisplay_window (window=21548085, just_this_one_p=true) at xdisp.c:16695
#6  0x00000000004665a3 in redisplay_window_1 (window=21548085) at xdisp.c:14494
#7  0x000000000061b092 in internal_condition_case_1 (bfun=0x466561 <redisplay_window_1>, 
    arg=21548085, handlers=14582547, hfun=0x4664db <redisplay_window_error>) at eval.c:1338
#8  0x00000000004657c2 in redisplay_internal () at xdisp.c:14119
#9  0x0000000000462ca2 in redisplay () at xdisp.c:13254
#10 0x00000000005768ea in read_char (commandflag=1, map=26212659, prev_event=0, 
    used_mouse_menu=0x7fffffffe36f, end_time=0x0) at keyboard.c:2477
#11 0x00000000005866f7 in read_key_sequence (keybuf=0x7fffffffe520, bufsize=30, prompt=0, 
    dont_downcase_last=false, can_return_switch_frame=true, fix_current_buffer=true, 
    prevent_redisplay=false) at keyboard.c:9063
#12 0x000000000057386d in command_loop_1 () at keyboard.c:1365
#13 0x000000000061aff8 in internal_condition_case (bfun=0x573431 <command_loop_1>, handlers=19056, 
    hfun=0x572ac3 <cmd_error>) at eval.c:1314
#14 0x0000000000573073 in command_loop_2 (ignore=0) at keyboard.c:1107
#15 0x000000000061a5c7 in internal_catch (tag=45888, func=0x57304a <command_loop_2>, arg=0)
    at eval.c:1079
#16 0x0000000000573015 in command_loop () at keyboard.c:1086
#17 0x00000000005725b3 in recursive_edit_1 () at keyboard.c:692
#18 0x00000000005727b3 in Frecursive_edit () at keyboard.c:763
#19 0x0000000000570560 in main (argc=5, argv=0x7fffffffe9b8) at emacs.c:1626

Lisp Backtrace:
"redisplay_internal (C function)" (0x0)

I see the same failure *without* modifying the :align-to expression on
Emacs 24.5.