The un-hidden Ghost shadow in 16 bit display
If your monitor display setting is 16 bit rather than 24 bit, then you would possibly see the un-hidden ghost shadow when you scroll the masked text. Try press the arrows to scroll the masked text and feel what is the "ghost shadow"; We can see text characters faintly moving behind the bitmap.
Some may feel annoying when their customer complains.
It is un-practical to warn every customer that they should set display as 24 bits. So, how to fix it ?
This phenomenon is a side effect of bitmap shift. So the solution is try to make small change to the background image. Modify its brightness, alpha, width or height will sometimes correct the ghost shadow.
For example, I modify the width of the bitmap by stretch it 2 pixels, then the problem is solved. In fact, the original movie does not produce ghost shadow. It produce that ghost shadow because I shrink the width by 2 pixels. The movie below is the one without ghost shadow.