I can now recreate at will with a magit sequence (l o hackbox/ TAB) which triggers a minibuffer re-size to accommodate the list of git branches:
(gdb) info frame 0
Stack frame at 0x7fffffffb2e0:
rip = 0x5555556a80ef in Factive_minibuffer_window (minibuf.c:230); saved rip = 0x5555556f52ab
called by frame at 0x7fffffffb340
source language c.
Arglist at 0x7fffffffb2c8, args:
Locals at 0x7fffffffb2c8, Previous frame's sp is 0x7fffffffb2e0
Saved registers:
rip at 0x7fffffffb2d8
(gdb) x/5i $pc
=> 0x5555556a80ef <Factive_minibuffer_window+79>: mov -0x3(%rax),%r10
0x5555556a80f3 <Factive_minibuffer_window+83>: lea -0x3(%rdx),%eax
0x5555556a80f6 <Factive_minibuffer_window+86>: test $0x7,%al
0x5555556a80f8 <Factive_minibuffer_window+88>: jne 0x5555556a8153 <Factive_minibuffer_window+179>
0x5555556a80fa <Factive_minibuffer_window+90>: nopw 0x0(%rax,%rax,1)
(gdb) p/x $rax
$4 = 0x0
(gdb) p/x $r10
$5 = 0x7fffeece9c6d
(gdb) l
225 Lisp_Object innermost_MB;
226
227 if (!minibuf_level)
228 return Qnil;
229
230 innermost_MB = nth_minibuffer (minibuf_level);
231 FOR_EACH_FRAME (frames, frame)
232 {
233 f = XFRAME (frame);
234 if (FRAME_LIVE_P (f)
(gdb) p minibuf_level
$6 = 2
(gdb) p Vminibuffer_list
$7 = (Lisp_Object) 0x555555c9aca3
(gdb) p $*
A syntax error in expression, near `'.
(gdb) p *$
$8 = <incomplete type>
(gdb)
Let me know if you want something else.