SafeArrayAccessData and SafeArrayAddRef?
Two ways of preserving the data.
scope_exit for running code at scope exit. C# says “We have scope_exit at home.”
You can wrap it in an IDisposable.
It doesn't do much, but it saves typing.
You are replacing the window procedure, not the dialog procedure.
The tab index number is an authoring concept, not a runtime concept.
The strings are an implementation detail.
Finding all the clipping parents.
Finding the correct clipping parent.
Preventing the cursor from pointing to nothing.
Looking at child objects.