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;
 }
 
@@ -1670,9 +1691,9 @@ JS_SetContextDebugHooks(JSContext *cx, JSDebugHooks *hooks)
 #ifdef MOZ_SHARK
 
 JS_PUBLIC_API(JSBool)
-JS_StartChudRemote() 
+JS_StartChudRemote()
 {
-    if (chudIsRemoteAccessAcquired() && 
+    if (chudIsRemoteAccessAcquired() &&
         (chudStartRemotePerfMonitor("Mozilla") == chudSuccess)) {
         return JS_TRUE;
     }
@@ -1730,7 +1751,7 @@ js_StopShark(JSContext *cx, JSObject *obj,
     if (!JS_StopChudRemote()) {
         JS_ReportError(cx, "Error stopping CHUD.");
     }
-        
+
     return JS_TRUE;
 }
 
@@ -1741,7 +1762,7 @@ js_ConnectShark(JSContext *cx, JSObject *obj,
     if (!JS_ConnectShark()) {
         JS_ReportError(cx, "Error connecting to Shark.");
     }
-        
+
     return JS_TRUE;
 }
 
@@ -1752,7 +1773,7 @@ js_DisconnectShark(JSContext *cx, JSObject *obj,
     if (!JS_DisconnectShark()) {
         JS_ReportError(cx, "Error disconnecting from Shark.");
     }
-        
+
     return JS_TRUE;
 }