Fix dropped and duplicated lines for nested collapse ranges - #470

Summary

collapse={3-5, 1-8} renders code that no longer matches the source. On a 10-line block, line 10 disappears and lines 3 to 5 show up a second time at the bottom. Writing the same two ranges the other way round, collapse={1-8, 3-5}, renders correctly, so whether you lose a line depends on the order you list the ranges in.

Cause

parseSections in packages/@expressive-code/plugin-collapsible-sections/src/utils.ts skips any range that overlaps a section it already collected, because the <details>-based rendering cannot handle overlapping sections. The check (lines 23 to 26 before this change) only tested whether the new range's from or to falls inside an existing section:

if (from >= existingFrom && from <= existingTo) return
if (to >= existingFrom && to <= existingTo) return

A range that fully contains an existing section satisfies neither test, so { from: 1, to: 8 } was collected alongside { from: 3, to: 5 } instead of being skipped.

sectionizeAst in src/ast.ts then splices both sections into the output array from last to first (line 57), while reading their contents from the unmodified lines array. Splicing the 1-8 section replaces eight entries with one, so the next outp.splice(from - 1, to - from + 1, outerElement) for 3-5 lands on entries that have shifted: it removes line 10 and appends a second copy of lines 3 to 5.

Fix

The two partial checks become the standard interval overlap test, which covers containment in both directions:

if (from <= existingTo && existingFrom <= to) return

Every range combination that already worked keeps its previous result. collapse={2-5, 3-6}, collapse={2-5, 5-6} and collapse={2-5, 1-2} still drop the second range, and collapse={2-5, 6-10} still keeps both.

Tests

test/preprocess-meta.test.ts gets the two orderings of a containing range, and test/rendering.test.ts gets a rendering test that asserts the section boundaries and that the rendered code lines are exactly the source lines, so a dropped or duplicated line fails the test. Both fail on main and pass with the fix.

Verification

pnpm --filter @expressive-code/plugin-collapsible-sections test-short passes, 14 tests in 2 files. With src/utils.ts reverted to main and the new tests in place, the same command fails 2 of 14, reporting sections at lines 1-9 and 10-12 for a 9-line block.

pnpm --filter @expressive-code/core --filter @expressive-code/plugin-collapsible-sections --filter @expressive-code/plugin-frames --filter @expressive-code/plugin-line-numbers --filter @expressive-code/plugin-text-markers test-short passes, 459 tests across the five packages.

pnpm exec eslint packages/@expressive-code/plugin-collapsible-sections and pnpm exec prettier --check on the changed files are both clean, and pnpm --filter @expressive-code/plugin-collapsible-sections build succeeds.

Added a patch changeset for @expressive-code/plugin-collapsible-sections.