The option buffered_drawing is by default set to 1, which results in single-buffered windows:

# Change the buffering scheme for drawing
# 0 no double buffering - useful for debugging
# 1 light buffering using a single back buffer for all windows
# 2 full fltk-based double buffering for all windows
#buffered_drawing=1

We may want to move the default to double buffering if there are no further issues, so we remove the artifacts on resizing the Dillo window.