Impossible to render certain Chinese fonts at low pixel size but others work fine

[ctrlcctrlv]

By impossible, I mean the computer freezes, not it can't be done—I-perf, not I-crash, in tag terms. The more glyphs that are on the screen, the slower it renders.

I discovered this bug while going through a collection of Chinese fonts I found, hitting «Encoding→Compact» and scrolling.

Many fonts in the collection are affected. Among the affected fonts is 蒙纳繁版黑.otf (CHei3HK-Bold-Roman).

I'm still working out what the problem is; whatever it is, it's deep in the system. A hint is that if I kill FontForge in GDB, the window will still render eventually, meaning it seems to be waiting on Xorg to respond.

Here's a traceback:

0x0000155553b22628 in poll () from /usr/lib/libc.so.6
(gdb) bt
#0  0x0000155553b22628 in poll () at /usr/lib/libc.so.6
#1  0x00001555520f2630 in  () at /usr/lib/libxcb.so.1
#2  0x00001555520f2d21 in  () at /usr/lib/libxcb.so.1
#3  0x00001555520f2dae in xcb_writev () at /usr/lib/libxcb.so.1
#4  0x000015555269ecdf in _XSend () at /usr/lib/libX11.so.6
#5  0x00001555520cc7d9 in XRenderAddGlyphs () at /usr/lib/libXrender.so.1
#6  0x0000155553e1291a in  () at /usr/lib/libcairo.so.2
#7  0x0000155553df48c2 in  () at /usr/lib/libcairo.so.2
#8  0x0000155553df4df7 in  () at /usr/lib/libcairo.so.2
#9  0x0000155553d9c5c1 in  () at /usr/lib/libcairo.so.2
#10 0x0000155553e17160 in  () at /usr/lib/libcairo.so.2
#11 0x0000155553dee6c9 in  () at /usr/lib/libcairo.so.2
#12 0x0000155553da6082 in  () at /usr/lib/libcairo.so.2
#13 0x0000155553dfb342 in cairo_show_glyphs () at /usr/lib/libcairo.so.2
#14 0x000015555401e6e4 in  () at /usr/lib/libpangocairo-1.0.so.0
#15 0x000015555401eb5f in  () at /usr/lib/libpangocairo-1.0.so.0
#16 0x0000155553fd8e3b in pango_renderer_draw_glyphs () at /usr/lib/libpango-1.0.so.0
#17 0x000015555401aafa in pango_cairo_show_glyph_string () at /usr/lib/libpangocairo-1.0.so.0
#18 0x0000555555772db3 in _GGDKDraw_MyCairoRenderLayout
    (cc=0x555555aaf450, fg=<optimized out>, layout=<optimized out>, x=1203, y=474)
    at gdraw/ggdkcdraw.c:230
#19 0x00005555557729f1 in GGDKDrawDoText8
    (w=0x555555d05450, x=1203, y=474, text=0x7fffffffc430 "獑", cnt=-1, col=2236962, drawit=tf_drawit, arg=0x7fffffffc358) at gdraw/ggdkcdraw.c:1259
--Type <RET> for more, q to quit, c to continue without paging--
#20 0x000055555572e7d8 in GDrawDrawText8
    (gw=0x7fffffff9c08, x=1, y=-1, text=0x155553b22628 <poll+24> "H=", cnt=1436603336, col=0)
    at gdraw/gdrawtxt.c:125
#21 0x000055555566da0a in FVExpose
    (fv=<optimized out>, pixmap=<optimized out>, event=<optimized out>)
    at fontforgeexe/fontview.c:6005
#22 0x000055555566da0a in v_e_h (gw=<optimized out>, event=<optimized out>)
    at fontforgeexe/fontview.c:6748
#23 0x000055555572a7b2 in _GWidget_Container_eh (gw=0x555555d05450, event=0x7fffffffc7b0)
    at gdraw/gcontainer.c:281
#24 0x0000555555737494 in _GGDKDraw_CallEHChecked (gw=<optimized out>, event=0x7, eh=
    0xfffffffffffffdfc) at gdraw/ggdkdraw.c:323
#25 0x0000555555737494 in _GGDKDraw_DispatchEvent
    (event=<optimized out>, data=<optimized out>) at gdraw/ggdkdraw.c:1221
#26 0x00001555542cf8b4 in  () at /usr/lib/libgdk-3.so.0
#27 0x00001555542c2142 in  () at /usr/lib/libgdk-3.so.0
#28 0x00001555542c20bc in  () at /usr/lib/libgdk-3.so.0
#29 0x00001555542c1b2f in  () at /usr/lib/libgdk-3.so.0
#30 0x00001555542c1cfd in  () at /usr/lib/libgdk-3.so.0
#31 0x0000155553d3eb4a in g_signal_emit_valist () at /usr/lib/libgobject-2.0.so.0
#32 0x0000155553d3f7f0 in g_signal_emit () at /usr/lib/libgobject-2.0.so.0
#33 0x00001555542cbad0 in  () at /usr/lib/libgdk-3.so.0
#34 0x00001555542da35c in  () at /usr/lib/libgdk-3.so.0
#35 0x0000155553c5fa74 in  () at /usr/lib/libglib-2.0.so.0
--Type <RET> for more, q to quit, c to continue without paging--
#36 0x0000155553c6027f in g_main_context_dispatch () at /usr/lib/libglib-2.0.so.0
#37 0x0000155553c621c1 in  () at /usr/lib/libglib-2.0.so.0
#38 0x0000155553c62201 in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#39 0x000055555573a4e6 in GGDKDrawEventLoop (gdisp=0x555555a69e90) at gdraw/ggdkdraw.c:2187
#40 0x00005555557084f8 in fontforge_main (argc=<optimized out>, argv=<optimized out>)
    at fontforgeexe/startui.c:1416
#41 0x0000155553a57ee3 in __libc_start_main () at /usr/lib/libc.so.6
#42 0x00005555555aba0e in _start ()

[ctrlcctrlv]

By the way, here's an absolutely bizarre hint. If I go «Ctrl-A», then Hints menu, Clear Hints, in the FontView, and wait for that to happen, scrolling becomes instant.

This collection is really old, ©2005, so I'm going to just share an affected font and let the chips fall where they may. If we solve this I'll remove it.

蒙纳简版黑.zip