Once you make it possible to do something, you have to accept that you also made it possible to do something wrong.

When the window manager was originally designed, it made it possible for programs to override many standard behaviors. They could handle the WM_NC­HIT­TEST message so a window can be dragged by grabbing any part of the window, not just the caption bar. They could handle the WM_NC­PAINT message to draw custom title bars. The theory was that making all of these things possible permitted smart people to do clever things.

The downside is that it also permits stupid people to do dumb things.

Changing the window procedure model from call Def­Window­Proc to get default behavior to return whether you handled the message wouldn't have helped. First of all, the handled/not-handled model is too restrictive: It requires you to do everything (handled) or nothing (not handled). There is no option to do a little bit. (Imagine if C++ didn't let you call the base class implementation of an overridden method.)

Doing a little bit is a very common pattern. The WM_NC­HITTEST technique mentioned above, for example, uses the default hit-testing implementation, and then tweaks the result slightly:

case WM_NCHITTEST:
 // call base class first
 lres = DefWindowProc(hwnd, uMsg, wParam, lParam);
 // tweak the result
 if (lres == HTCLIENT) lres = HTCAPTION;
 return lres;

How would you do this with the handled/not-handled model?

case WM_NCHITTEST:
 if (not handling this message would have resulted in HTCLIENT) {
  lres = HTCAPTION;
  handled = TRUE;
 } else {
  handled = FALSE;
 }
 break;

The trick about that bit in parentheses is that it requires the research department to finish the final details on that time machine they've been working on. It's basically saying, "Return not handled, then follow the message until handling is complete and if the final result is HTCLIENT, then fire up the time machine and rewind to this point so I can change my mind and return handled instead."

And even if the research department comes through with that time machine, the handled/not-handled model doesn't even solve the original problem!

The original problem was people failing to call Def­Window­Proc when they decided that they didn't want to handle a message. In the handled/not-handled model, the equivalent problem would be people returning handled = TRUE unconditionally.

BOOL NewStyleWindowProc(HWND hwnd, UINT uMsg,
 WPARAM wParam, LPARAM lParam, LRESULT& lres)
{
 BOOL handled = TRUE;
 switch (uMsg) {
 case WM_THIS: ...; break;
 case WM_THAT: ...; break;
 // no "default: handled = FALSE; break;"
 }
 return handled;
}

(Side note: The dialog manager uses the handled/not-handled model, and some people would prefer that it use the Def­Xxx­Proc model, so you might say "We tried that, and some people didn't like it.")

This topic raises another one of those "No matter what you do, somebody will call you an idiot" dilemmas. On the one side, there's the Windows should perform extra testing at runtime to detect bad applications school, and on the other side, there's the Windows should get rid of all the code whose sole purpose in life is to detect bad applications school.