bug 753274. Make `nsWindow::SubclassWindow` only fail if `mWnd` is an invalid window AND we're trying to apply a subclass (rather than remove a subclass). r=jimm

widget/windows/nsWindow.cpp

@@ -850,34 +850,43 @@ DWORD nsWindow::WindowExStyle()
 // Subclass (or remove the subclass from) this component's nsWindow
 void nsWindow::SubclassWindow(BOOL bState)
 {
-  if (NULL != mWnd) {
-    //NS_PRECONDITION(::IsWindow(mWnd), "Invalid window handle");
-    if (!::IsWindow(mWnd)) {
+  if (bState) {
+    if (!mWnd || !IsWindow(mWnd)) {
       NS_ERROR("Invalid window handle");
     }
 
-    if (bState) {
-      // change the nsWindow proc
-      if (mUnicodeWidget)
-        mPrevWndProc = (WNDPROC)::SetWindowLongPtrW(mWnd, GWLP_WNDPROC,
-                                                (LONG_PTR)nsWindow::WindowProc);
-      else
-        mPrevWndProc = (WNDPROC)::SetWindowLongPtrA(mWnd, GWLP_WNDPROC,
-                                                (LONG_PTR)nsWindow::WindowProc);
+    if (mUnicodeWidget) {
+      mPrevWndProc =
+        reinterpret_cast<WNDPROC>(
+          SetWindowLongPtrW(mWnd,
+                            GWLP_WNDPROC,
+                            reinterpret_cast<LONG_PTR>(nsWindow::WindowProc)));
+    } else {
+      mPrevWndProc =
+        reinterpret_cast<WNDPROC>(
+          SetWindowLongPtrA(mWnd,
+                            GWLP_WNDPROC,
+                            reinterpret_cast<LONG_PTR>(nsWindow::WindowProc)));
+    }
     NS_ASSERTION(mPrevWndProc, "Null standard window procedure");
     // connect the this pointer to the nsWindow handle
     WinUtils::SetNSWindowPtr(mWnd, this);
+  } else {
+    if (IsWindow(mWnd)) {
+      if (mUnicodeWidget) {
+        SetWindowLongPtrW(mWnd,
+                          GWLP_WNDPROC,
+                          reinterpret_cast<LONG_PTR>(mPrevWndProc));
+      } else {
+        SetWindowLongPtrA(mWnd,
+                          GWLP_WNDPROC,
+                          reinterpret_cast<LONG_PTR>(mPrevWndProc));
+      }
     }
-    else {
-      if (mUnicodeWidget)
-        ::SetWindowLongPtrW(mWnd, GWLP_WNDPROC, (LONG_PTR)mPrevWndProc);
-      else
-        ::SetWindowLongPtrA(mWnd, GWLP_WNDPROC, (LONG_PTR)mPrevWndProc);
     WinUtils::SetNSWindowPtr(mWnd, NULL);
     mPrevWndProc = NULL;
   }
 }
-}
 
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