fix: fixes error when a utility frame is the current frame

On Mac OS X, small frames that don't seem to have any windows may be
selected when the process filter is processed.  By walking up the
parent-frame list, we can get to the frame that the user is actually
able to use, and thus the call to (get-mru-window) does not return nil
and the whole process filter aborts.

Should fix bug #828

Author: ryanobjc

dap-mode.el

@@ -784,7 +784,7 @@ will be reversed."
   (-when-let* ((source (gethash "source" stack-frame))
                (sourceReference (gethash "sourceReference" source))
                (sourceReferenceKey (format "%s-%s" name sourceReference)))
-    (select-window (get-mru-window (selected-frame) nil))
+    (select-window (get-mru-window (dap--selected-frame) nil))
     (if-let* ((existing-buffer (get-buffer sourceReferenceKey)))
         (switch-to-buffer existing-buffer)
       (dap--send-message
@@ -810,6 +810,15 @@ will be reversed."
                          path)))
       (--> debug-session (dap--debug-session-remote-to-local-path-fn it) (funcall it remote-path)))))
 
+(defun dap--selected-frame ()
+  "Select the 'main' frame when we have a utility/subframe selected"
+  (cl-labels ((helper (f)
+                (let ((parent-frame (frame-parameter f 'parent-frame)))
+                  (if parent-frame
+                      (helper parent-frame)
+                    f))))
+    (helper (selected-frame))))
+
 (defun dap--go-to-stack-frame (debug-session stack-frame)
   "Make STACK-FRAME the active STACK-FRAME of DEBUG-SESSION."
   (when stack-frame
@@ -821,7 +830,7 @@ will be reversed."
       ;; stack trace.
       (if (and path (file-exists-p path))
           (progn
-            (select-window (get-mru-window (selected-frame) nil))
+            (select-window (get-mru-window (dap--selected-frame) nil))
             (find-file path)
             (goto-char (point-min))
             (forward-line (1- line))