mrbkap's patch for 418293 with a few nits picked, r=me, a=beltzner.

js/src/jsdbgapi.c

@@ -1085,11 +1085,32 @@ JS_GetFrameCallObject(JSContext *cx, JSStackFrame *fp)
 JS_PUBLIC_API(JSObject *)
 JS_GetFrameThis(JSContext *cx, JSStackFrame *fp)
 {
-    if (!fp->thisp && fp->argv) {
-        fp->thisp = js_ComputeThis(cx, JS_TRUE, fp->argv);
-        if (!fp->thisp)
-            return NULL;
+    JSStackFrame *afp;
+
+    if (fp->flags & JSFRAME_COMPUTED_THIS)
+        return fp->thisp;
+
+    /* js_ComputeThis gets confused if fp != cx->fp, so set it aside. */
+    if (cx->fp != fp) {
+        afp = cx->fp;
+        if (afp) {
+            afp->dormantNext = cx->dormantFrameChain;
+            cx->dormantFrameChain = afp;
+            cx->fp = fp;
         }
+    } else {
+        afp = NULL;
+    }
+
+    if (!fp->thisp && fp->argv)
+        fp->thisp = js_ComputeThis(cx, JS_TRUE, fp->argv);
+
+    if (afp) {
+        cx->fp = afp;
+        cx->dormantFrameChain = afp->dormantNext;
+        afp->dormantNext = NULL;
+    }
+
     return fp->thisp;
 }