What happens if you change a window class’s GCL_CB­WND­EXTRA?

This article discusses the implications of modifying the GCL_CB­WND­EXTRA value for window classes in Windows. It explains how changes affect newly created windows but not existing ones, leading to potential confusion when accessing extra bytes.

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After my historical look back on the evolution of system-windows window and class extra bytes , I noted that there was one application that expected to be able to modify GWW_ CB­CLS­EXTRA .

It turns out that there are even more applications that expect to be able to modify GWW_ CB­WND­EXTRA . So many that it wasn’t worth creating an application compatibility exception for them.

So what happens when you modify GWW_ CB­WND­EXTRA , or its modern equivalent, GWL_ CB­WND­EXTRA ?

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