You can never count on the backbuffer remaining in place after a flip...
On some video cards the screen will be automatically cleared, and on others you may either see the previous frame, or whatever was on the screen 2, 3, or 4 frames ago. (Which leads to horrible flickering and jumping around.
Different video card drivers treat the backbuffers differently, and may recycle anywhere from 0 to 4 or more of them. As you found out, even full-screen and windowed don't behave the same on all computers...
bottom line: Don't count on the backbuffer showing the previous frame, ever. Many, many computers will return unpredictable results.
On some video cards the screen will be automatically cleared, and on others you may either see the previous frame, or whatever was on the screen 2, 3, or 4 frames ago. (Which leads to horrible flickering and jumping around.
Different video card drivers treat the backbuffers differently, and may recycle anywhere from 0 to 4 or more of them. As you found out, even full-screen and windowed don't behave the same on all computers...
bottom line: Don't count on the backbuffer showing the previous frame, ever. Many, many computers will return unpredictable results.