[22.1.90]: Barf in left margin in *stack frames of emacs*

["Marshall, Simon" <Simon.Marshall@misys.com>]

In *stack frames of emacs*, I can left-click in the right margin to
horizontally scroll the window.  Gud is good.  However, when I
left-click in the left margin to horizontally scroll back, I get an
error.

emacs -Q
M-x gdb RET RET	; for me, running gdb on emacs
In *gud-emacs*, put a breakpoint somewhere that will get hit and "r -Q"

When the breakpoint hits, select "Options | Enter Debugger on Error" and
"Gud | GDB-Frames | Stack" to popup the *stack frames of emacs* frame.
Left-click in the right margin to horizontally scroll the *stack frames
of emacs* window, then left-click in the left margin to scroll back.  I
get:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  file-name-nondirectory(nil)
  gud-format-command("break %f:%l" nil)
  gud-call("break %f:%l" nil)
  gud-break(nil)
  gdb-mouse-set-clear-breakpoint((mouse-1 (#<window 27 on *stack frames
of emacs*> left-fringe (2 . 32) 178127836 nil 250 (0 . 2) nil (2 . 6) (8
. 13))))
  call-interactively(gdb-mouse-set-clear-breakpoint)

It shouldn't signal an error, but it appears to be trying to do
something which is not what I would expect it to do.  I would not expect
to be setting breakpoints from the stack frame window in any case.


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