overlay inherited opacity:1 from .lightbox-overlay CSS, so the moment
it was appended to the DOM it painted a full-opacity black background.
this showed for ~24ms (the time until the image loaded and start() ran
the first frame that sets opacity 0), visible as a black flash before
the fade-in.
set overlay.style.opacity = '0' right after creating it, so it is
transparent when appended; start()'s transition then fades it to 1 as
intended.
xfy
c1610ace99