On Thu, Apr 25 2024, Gerd Möllmann wrote:

>> ./src/emacs -Q src/xdisp.c -eval '(igc--collect)'
>
> I would start by investigating what stuff reachable from struct frame
> is and is not traced, which depends a bit on the platform. Also font
> stuff probably, terminals, scroll bar things (at least in NS I know
> there is something), and I don't know what else.

The problem was a corrupted font.  I added the code below and it now
survives the first GC cycle.  In general, fix_frame must scan the same
fields as mark_frame in alloc.c does.  Is that about right?  Since MPS
wants the address of the fields (instead the value), some creativity may
be required.  Is that also right?  Or can I stupidly copy what
mark_frame does?

Helmut