Holy cow, I wrote a book!
The special values
HWND_TOP and HWND_TOPMOST
have similar names but do completely different things when passed as
hWndInsertAfter parameter to
the DeferWindowPos function
(or its moral equivalents such as SetWindowPos).
As a backgrounder, you should start off by reading
the MSDN discussion, which is perfectly accurate as far as it goes.
Here, I'll discuss the issue from a historical perspective
in the hopes that looking at it from a different direction
may improve understanding.
Sibling windows are maintained in an order called the Z-order.
(For the purpose of this discussion,
top-level windows are also treated as siblings.
In fact, it is the Z-order of top-level windows
that most people think of when they say "Z-order".)
The Z-order should be visualized as a vertical stack,
with windows "above" or "below" siblings.
Before Windows 3.0,
the behavior was simple:
brings the window to the top of the Z-order.
Windows 3.0 added the concept of "topmost" windows.
These are top-level windows that always remain "above" non-topmost windows.
To make a window topmost, call
DeferWindowPos (or one of its moral equivalents)
with HWND_TOPMOST as the hWndInsertAfter.
To make a window non-topmost, use HWND_NOTOPMOST.
As a result of the introduction of "topmost" windows,
HWND_TOP now brings the window
"as high in the Z-order as possible
without violating the rule that topmost windows
always appear above non-topmost windows".
What does this mean in practice?
Note: The above discussion completely ignores the issue of owner
and owned windows.
I left them out because they would add a
layer of complication that distracts from the main topic.