Subject: PDF export adds a thin light strip at the top (and sometimes the bottom) of every page

What happens

When I export a 16:9 deck to PDF, every page has a thin off-white line along its top edge. It's most visible on dark slides, where it shows as a white hairline. Some pages also have a 1px sliver along the bottom.

What I measured in the exported file

Page size is 900 × 507.12 pt. A true 16:9 page at that width is 900 × 506.25 pt, so each page is about 0.87pt too tall.

At 300 dpi, the top 2–3 pixel rows (about 0.7pt) of every page are off-white (RGB 246, 244, 244) before the slide content starts.

At 300 dpi, the bottom row is white on pages 2, 4, 5 and 6 of my export, which are dark slides.

It appears on all 13 pages, white slides included, so it isn't caused by any one slide's background.

What I tried

I set a dark card-level backing colour on the dark and photo slides. The line was still in the PDF, so it comes from the page size and not from the slide design.

Expected

Pages exactly 16:9, with slide content filling the page edge to edge.

Workaround for now

Cropping about 0.7pt off the top and 0.1pt off the bottom of each page removes the line.