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Toggling on matrix tripped up address sanitizer:

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==24956==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffcd88055a2 at pc 0x7f59c0e51a90 bp 0x7ffcd8805460 sp 0x7ffcd8804c10
WRITE of size 3 at 0x7ffcd88055a2 thread T0

    #0 0x7f59c0e51a8f in strncpy (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x55a8f)
    #1 0x7f59c0b49357 in strncpy /usr/include/x86_64-linux-gnu/bits/string_fortified.h:106
    #2 0x7f59c0b49357 in g_utf8_strncpy ../../../glib/gutf8.c:441
    #3 0x55e49892f29f in chatty_icon_get_buddy_icon ../src/chatty-icons.c:230
    #4 0x55e498921347 in chatty_conv_muc_add_user ../src/chatty-conversation.c:1530
    #5 0x55e498921748 in chatty_conv_muc_list_add_users ../src/chatty-conversation.c:1617
    #6 0x7f59c00b1a84 in purple_conv_chat_add_users ./libpurple/conversation.c:1746
    #7 0x7f59b4d76922 in matrix_room_complete_state_update (/usr/lib/purple-2/libmatrix.so+0x9922)
    #8 0x7f59b4d778f1 in matrix_sync_parse (/usr/lib/purple-2/libmatrix.so+0xa8f1)
    #9 0x7f59b4d747d5  (/usr/lib/purple-2/libmatrix.so+0x77d5)
    #10 0x7f59b4d737a9  (/usr/lib/purple-2/libmatrix.so+0x67a9)
    #11 0x7f59c00ffc4c in url_fetch_recv_cb ./libpurple/util.c:3996
    #12 0x55e49892dc60 in purple_glib_io_invoke ../src/chatty-purple-init.c:67
    #13 0x7f59c0b17dd7 in g_main_dispatch ../../../glib/gmain.c:3182
    #14 0x7f59c0b17dd7 in g_main_context_dispatch ../../../glib/gmain.c:3847
    #15 0x7f59c0b181c7 in g_main_context_iterate ../../../glib/gmain.c:3920
    #16 0x7f59c0b1825b in g_main_context_iteration ../../../glib/gmain.c:3981
    #17 0x7f59c0d0a99c in g_application_run ../../../gio/gapplication.c:2470
    #18 0x55e498911727 in main ../src/main.c:43
    #19 0x7f59bfe8a09a in __libc_start_main ../csu/libc-start.c:308
    #20 0x55e498911359 in _start (/var/scratch/librem5/chatty/_build/src/chatty+0x1a359)

Address 0x7ffcd88055a2 is located in stack of thread T0 at offset 34 in frame
    #0 0x55e49892eba3 in chatty_icon_get_buddy_icon ../src/chatty-icons.c:116

  This frame has 5 object(s):
    [32, 34) 'tmp' <== Memory access at offset 34 overflows this variable
    [96, 100) 'scale_width'
    [160, 164) 'scale_height'
    [224, 232) 'len'
    [288, 336) 'te'
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x55a8f) in strncpy
Shadow bytes around the buggy address:
  0x10001b0f8a60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10001b0f8a70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10001b0f8a80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10001b0f8a90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10001b0f8aa0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x10001b0f8ab0: f1 f1 f1 f1[02]f2 f2 f2 f2 f2 f2 f2 04 f2 f2 f2
  0x10001b0f8ac0: f2 f2 f2 f2 04 f2 f2 f2 f2 f2 f2 f2 00 f2 f2 f2
  0x10001b0f8ad0: f2 f2 f2 f2 00 00 00 00 00 00 f2 f2 f3 f3 f3 f3
  0x10001b0f8ae0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10001b0f8af0: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
  0x10001b0f8b00: 00 00 00 00 f3 f3 f3 f3 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==24956==ABORTING

This looks related to #103 (closed) from your description.