There is an extended window style known as
WS_.
Explaining what this style does
is kind of difficult,
but it's been on the list of future topics
since 2003.
Better late than never, I guess.
First of all, there are two unrelated senses of transparent in play. One is visual transparency: Pixels beneath the window can show through. The other is hit-test transparency: Do clicks on this window go to the window, or do they pass through to the window beneath?
And each of those senses of transparent is itself complicated.
Let's look a hit-test transparency first.
We saw some time ago that
It's Complicated.
Different hit-testing functions use different methods of
determining whether a click should be given to a window
or pass through it.
And the
WS_ extended window style
plays into some of those rules.
I'll let you read the linked article to learn the details.
Okay, now to visual transparency.
The
WS_ extended window style
alters the painting algorithm as follows:
If a
WS_ window
needs to be painted, and it has any
non-WS_ windows
siblings (which belong to the same process)
which also need to be painted,
then the window manager will paint the
non-WS_ windows
first.
That "lets the other windows paint first" behavior is what gave the style its name, because if you let the other windows paint first, and then you paint on top of it, then the parts you don't actually paint will appear to be transparent because the other windows' pixels will show through.
Calling this painting style transparent is kind of overselling the feature. It's really a very specific behavior which makes transparent rendering sort of possible; it's not a just turn this on and you get transparent rendering feature.
It's like taking a lump of coal and labeling it diamond. Or like those advertisements which say things like "Can create up to five million cups of coffee per day!"
In order to turn the flour into a cake, the window with the
WS_ window style needs
to know not to draw opaquely.
Because if it draws opaquely, then all the work the window manager
did to get the other window to draw first (so you can draw
on top of it)
is for naught.
And sometimes the
WS_ window style doesn't
even work.
For example, observe that if the windows are not siblings of
each other, then the style has no effect.
If the windows belong to different processes,
then the style has no effect.
If both windows are not invalid simultaneously,
then the style has no effect.
(I don't know how you could get into that last state,
but maybe you're more creative than I am.)
Go ahead and use the
WS_ window style if that's
what you want.
Just understand that it doesn't get you what the name might promise.
(Depending on your situation, a layered window may work better.)