Transparent curtain in front of js-based lightboxes WP 3.6-beta3-24372

Depending on the browser, different js-based lightbox images show correctly for a brief moment but the are fully or partly covered by a black transparent “curtain”, which most probably should be behind the image to partly cover the actual page in the background.

Happens e.g. with Pinterest Lightbox plugin and PicLens when showing images from NextGEN Gallery. Effects on a couple of browsers:
– Opera 12.15: both the above lightboxes fully covered by the ‘curtain’
– Firefox 20.0.1: PicLens works ok, but Pinterest Lightbox images are halfway from top covered by this ‘curtain’.

With WP 3.5.1 all these work ok.

I would not use 3.6. in production unless this is somehow solved.