diff --git a/dom/plugins/PluginInstanceChild.cpp b/dom/plugins/PluginInstanceChild.cpp
index ab60999def1..802496e9038 100644
--- a/dom/plugins/PluginInstanceChild.cpp
+++ b/dom/plugins/PluginInstanceChild.cpp
@@ -662,6 +662,8 @@ PluginInstanceChild::SizePluginWindow(int width,
                                       int height)
 {
     if (mPluginWindowHWND) {
+        mPluginSize.x = width;
+        mPluginSize.y = height;
         SetWindowPos(mPluginWindowHWND, NULL, 0, 0, width, height,
                      SWP_NOZORDER | SWP_NOREPOSITION);
     }
@@ -697,6 +699,27 @@ PluginInstanceChild::PluginWindowProc(HWND hWnd,
 
     NS_ASSERTION(self->mPluginWindowHWND == hWnd, "Wrong window!");
 
+    // Adobe's shockwave positions the plugin window relative to the browser
+    // frame when it initializes. With oopp disabled, this wouldn't have an
+    // effect. With oopp, GeckoPluginWindow is a child of the parent plugin
+    // window, so the move offsets the child within the parent. Generally
+    // we don't want plugins moving or sizing our window, so we prevent these
+    // changes here.
+    if (message == WM_WINDOWPOSCHANGING) {
+      WINDOWPOS* pos = reinterpret_cast<WINDOWPOS*>(lParam);
+      if (pos && (!(pos->flags & SWP_NOMOVE) || !(pos->flags & SWP_NOSIZE))) {
+        pos->x = pos->y = 0;
+        pos->cx = self->mPluginSize.x;
+        pos->cy = self->mPluginSize.y;
+        LRESULT res = CallWindowProc(self->mPluginWndProc, hWnd, message, wParam,
+                                     lParam);
+        pos->x = pos->y = 0;
+        pos->cx = self->mPluginSize.x;
+        pos->cy = self->mPluginSize.y;
+        return res;
+      }
+    }
+
     // The plugin received keyboard focus, let the parent know so the dom is up to date.
     if (message == WM_MOUSEACTIVATE)
         self->CallPluginGotFocus();
diff --git a/dom/plugins/PluginInstanceChild.h b/dom/plugins/PluginInstanceChild.h
index 47af9102690..ef2ce58a1dc 100644
--- a/dom/plugins/PluginInstanceChild.h
+++ b/dom/plugins/PluginInstanceChild.h
@@ -51,6 +51,7 @@
 #include "nsTArray.h"
 #include "ChildAsyncCall.h"
 #include "ChildTimer.h"
+#include "nsRect.h"
 
 namespace mozilla {
 namespace plugins {
@@ -229,6 +230,7 @@ private:
     bool mNestedEventState;
     HWND mCachedWinlessPluginHWND;
     UINT_PTR mEventPumpTimer;
+    nsIntPoint mPluginSize;
 #endif
 
     friend class ChildAsyncCall;