I'm using Bluefish 0.9 with libgtk2.0-0 (ver 2.0.6-3 in Debian testing).
Bluefish developer thinks this is gtk2 bug.

If one places the cursor at the top of a file and then moves it rapidly
down the page by holding down the down arrow key, several copies of the
cursor can be left behind in the text.  These disappear by moving the
cursor position earlier in the text than the 'shadow', or by scrolling the
page with the scroll bar.

Experimentation suggests this might be something to do with the length of a
wrapped line in the editing window.  I have not observed the effect on
lines that are shorter than the window width, or those that are only
wrapped a little. Re-sizing the window alters whether a shadow is left on a
particular line.