This bug appeared in 3.4.
1. Enable manga mode
2. Enable "Space key uses smart scolling" and "Always
use smart scrolling as if in double-page mode"
3. Open something, adjust window so that the image is
wider than the window (-> horizontal scrollbar). Press
spacebar to scroll: works.
4. Adjust the window so that the image fits in the
window width-wise. Press spacebar to scroll: doesn't
work. Disable manga mode, try again: works.