The anchored fence regex from the upstream commits (kept here via
cherry-pick) fixed#85381 but required the opening ``` to sit at a
line start with <=3 spaces of indent, silently downgrading two shapes
that main renders as <pre> into escaped literal prose:
- mid-line-opened fences ("Here is the code: ```python")
- fences indented by 4+ spaces (list/blockquote-nested code)
Widen the anchors while keeping the inline-triple fix:
- the opening line may now carry any leading whitespace or lead-in
prose, as long as the opening ``` is the first triple-backtick run
on the line and the rest of the line is backtick-free (so an inline
pair like ```x``` can never end a line and open a fence);
- the closing fence may sit at any indent, not just 0-3 spaces.
The tempered prefix cannot skip past an earlier triple-backtick run,
so an inline pair followed later by a real fence no longer swallows
it (both main and the anchored regex mangled that mix in different
ways). The scar conformance vector reverts to main's committed value:
the mid-line-opened fence it encodes is protected again.
c82abc309a