Why do Tinyslideshow thumbnails disappear when page is zoomed smaller?

Question

I set up TinySlideshow to have 5 vertical thumbnails at the bottom, which looks fine. But when the page is zoomed-out in FireFox 8, Internet Explorer 8, and Chrome to make the page smaller in the browser window, there is a problem. (However, the problem doesn't happen in Opera).

The last thumbnail at the far right edge disappears (turns black) when the page size is zoomed smaller. If the page is zoomed  back to normal size again, the thumbnail re-appears again. Scrolling the screen larger doesn't make the thumbnail disappear.

It seems like the thumbnails are spread across 2 rows (the 2nd row being invisible, positioned directly beneath the first) and the thumbnail on the far right edge is always being droppped down onto this second line when the page gets zoomed smaller.

Any solutions to this?

Thanks!