I don't know why this call to send WM_INITIALUPDATE to our windows causes issues and wish I did. I found the call to send it to the input CWnd caused the issue while sending it to the descendants of the input CWnd did not cause the issue.

I finally gave up and set m_bAutoInitialUpdate to false. But why? What's going on?

After updating our app to version 20.1, I found that our docking panes would not size. Nor would pinning and unpinning result in a good display. I had to resize the frame window to see the auto-hide panel or to get the app frame to display. I found the reason was that in CXTPDockingPaneManager::RecalcFrameLayout the code now checks m_bInitialUpdateCalled and if that member is false, it would just return rather than calling RecalcLayout on the frame.

I fixed that by sending the WM_INITIALUPDATE message to the pane manager. But now, I am having a lot of other issues with docking pane client windows. Using spy++ I found that mouse messages aren't getting to our dialogs or to the task panel we have as a child of the client window of the pane it is in. Cursors aren't updating when the mouse enters a window, controls don't respond to mouse clicks ...

Now I have found that CXTPDockingPane::Attach is also sending WM_INITIALUPDATE to each CWnd (dialogs in our case) being attached. If I skip sending the messages to the input CWnd, everything works as it has for ever.

What's going on here?