Holy cow, I wrote a book!
If you specify the CS_SAVEBITS class style,
then the window manager will try to save the bits covered
by the window.
But the real question is why, because that is your guide to
using this power only for good, not for evil.
When a window whose class specifies the CS_SAVEBITS
class style is displayed,
the window manager takes a snapshot of the pixels on the screen
where the window will be displayed.
First, it asks the video card to store the pixels in
available off-screen video memory (fast).
If no video memory is available,
then the pixels will be stored in system memory (slower).
If the saved pixels
have not been discarded in the meantime (see below),
then when the window is hidden,
the saved pixels are copied back to
the screen and validated;
in other words, the pixels are marked as "good"
and no WM_PAINT message is generated.
What invalidates the saved pixels?
Anything that would cause those pixels to be out of sync with
what should be on the screen once the popup window is removed.
Here are some examples:
You get the idea.
If copying the saved pixels back to the screen would result
in an inconsistent display, then the saved pixels are discarded.
So how do you use this power for good and not for evil?
One consideration is that the region should cover a relatively
small portion of the screen,
because the larger the saved bitmap, the less likely it will fit
into available off-screen video memory,
which means the more likely it will have to travel across the
bus in a video-to-system-memory blit,
the dreaded "vid-sys blt" that game developers are well familiar with.
In the grand scheme of vid/sys blts, "vid-vid" is the fastest
(since the video card is very good at shuffling memory around
"sys-sys" is next best (since the motherboard can shuffle memory
around within itself, though it'll cost you CPU cache space),
"sys-vid" is in third place, and "vid-sys" is the worst:
Programs write to video memory much more often than
they read from it.
As a result, the bandwidth between the video card and
system memory is optimized for writing to video, not reading from it.
But the primary concern for deciding when to use the
CS_SAVEBITS window class style is not making
the window manager
go to all the trouble of saving the pixels, only to have to
throw them away.
A window that is a good candidate for the CS_SAVEBITS
style is therefore one that does not move,
covers a relatively small portion of the screen,
and is visible for only a short time.
That the window shouldn't move is obvious:
If the window moves, then the saved pixels are useless.
The other two rules of thumb try to minimize the opportunity
for another window to do something that invalidates the saved
By keeping the window small in area and putting it on the screen
for only a short time,
you keep the "target" small both spatially and temporally.
Consequently, the best candidates for CS_SAVEBITS
are menus, tooltips, and small dialogs,
since they aren't too big, they don't typically move around,
and they go away pretty quickly.
(Some people appear to be under the mistaken impression that
CS_SAVEBITS saves the bits of the window itself.
I don't know where people get this impression from since even
a modicum of experimentation easily demonstrates it to be false.
The Windows drawing model follows the principle of
Don't save anything you can recalculate.)