Make sure you disable the correct window for modal UI

Raymond Chen


Some time ago,
I was asked to look at two independent problems with people trying
to do modal UI manually.
Well, actually, when the issues were presented to me,
they weren’t described in quite that way.
They were more along the lines of,
“Something strange is happening in our UI. Can you help?”
Only in the discussion of the scenarios did it become
apparent that it was improper management of modal UI that
was the cause.

We already saw

one subtlety of managing modal UI manually
namely that you have to enable and disable the windows in the correct
That wasn’t the root of the problems I was looking at,
but enabling and disabling windows did play a major role.

When we

took a look at the dialog loop
the first steps involved manipulating the hwndParent
parameter to ensure that we enable and disable the correct window
at the correct time.

 if (hwndParent == GetDesktopWindow())
  hwndParent = NULL;
 if (hwndParent)
  hwndParent = GetAncestor(hwndParent, GA_ROOT);
 HWND hdlg = CreateDialogIndirectParam(hinst,
               lpTemplate, hwndParent, lpDlgProc,
 BOOL fWasEnabled = EnableWindow(hwndParent, FALSE);

In both cases, the first two “if” statements were missing.
We already saw

the danger of disabling the desktop window
which is what the first “if” statement protects against.
But the specific problem with modal UI was being caused by
the missing second “if” statement.

Both of the problems boiled down to somebody passing a child
window as the hwndParent and the code doing
manual modal UI failing to convert this window to a top-level window.
As a result, when they did the EnableWindow(hwndParent, FALSE),
they disabled a child window,
leaving the top-level window enabled.

The two problems had the same root cause but manifested themselves
The first problem led to strange behavior because

the user could still interact
with the top-level window since it was still enabled
Sure, a portion of the window was disabled (the portion
controlled by the child window passed as hwndParent),
but the caption buttons still worked, as did many of the other
controls on the window.

In the second case,
disabling the wrong window created a different problem:
When the modal UI was complete,
the window manager activated the top-level window that was the
owner of the modal window since that window was never disabled.
This caused the top-level window to receive a WM_ACTIVATE
which it handled by putting focus on the control that had focus
when the top-level window was deactivated.
Unfortunately, that window was the window that was passed
as the hwndParent,
which was disabled by mistake.
The attempt to restore focus failed,
and when the manual modal UI finally finished up and enabled
the child window, it was too late.
You wound up with focus nowhere and a dead keyboard.
This second problem was
reported as simply “SetFocus is not working.”
Only after peeling back a few layers (and application of some
psychic powers) did the root cause emerge.

Now, even though this was a subtle problem,
you already knew all the pieces that went into it since I had
covered them earlier.
And as for those psychic powers that I used?
It’s really not that magic.
In this case of psychic debugging, I worked backwards.
In response to the report that
SetFocus was not working,
the next set of questions was to determine why.
Is it a valid window handle?
Does the window belong to your thread?
Is it enabled?

Aha, the window isn’t enabled.
That’s when the customer also mentioned that they were doing
this inside a WM_ACTIVATE handler.
If you’re gaining activation, who were you gaining it from?
Oh, a modal dialog, you say?
One that you’re managing manually?
Once I discovered that they were trying to manage modal UI manually,
I suspected that they were disabling the wrong window,
since that fit all the symptoms and it’s something that people tend
to get wrong.

Most of what looks like psychic debugging is really just
knowing what people tend to get wrong.

Raymond Chen
Raymond Chen

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