fix(highlights): prevent gray decoration box in Cursor

[Yuki-zik]

Fixes #1

Problem

In Cursor (a VS Code fork) the default neon+glass tag highlight rendered a solid gray rounded background block. In VS Code the identical config renders the intended translucent "glass" box (faint tint + colored border, rounded corners). The reporter noted the todo-highlight extension had the same problem in Cursor.

Root cause

Known VS Code/Cursor decoration behavior (microsoft/vscode#175819, wayou/vscode-todo-highlight#434): a TextEditorDecorationType that carries border + borderRadius without an explicit, co-located backgroundColor makes the editor paint a default fallback background — which renders gray in dark themes. Cursor exhibits this; stock VS Code currently does not.

buildGlassDecorationOptions in src/highlights.js put borderRadius on the decoration's base rule while backgroundColor/border lived only in the light/dark (theme-scoped) rules. The base rule therefore had border-radius with no background, so Cursor gray-filled it. This also matches the repo's own debugging_journal.md lesson about not splitting style props between the top level and the light/dark sub-objects.

Glass is the only decoration carrying border/radius (buildTextDecorationOptions, getSubTagDecoration, buildMetaDecorationOptions set neither), so the fix is isolated.

Fix

Rewrite buildGlassDecorationOptions so each themed object is a self-contained glass box:

• Co-locate borderRadius into light/dark next to the border (dropped from the top level). borderRadius is a valid ThemableDecorationRenderOptions property, so this is API-safe and visually identical in VS Code.
• Guarantee an explicit backgroundColor on any themed object that has a border/borderRadius; when the config supplies none, fall back to transparent (the documented upstream workaround).

The default neon+glass translucent tint is preserved, so VS Code's appearance is unchanged; the transparent fallback only protects border-only/custom configs. The four-channel model, range types, and opacity controls are untouched.

Tests

• New test/highlights.decorations.tests.js (5 behavioral tests) asserts: no top-level borderRadius; borderRadius co-located with the border in light/dark; transparent fallback for border-only configs; the invariant that no glass theme has a border/borderRadius without an explicit background; and background-only configs are untouched.
• _buildGlassDecorationOptions is exported for testing (mirrors existing _-prefixed test exports).

Verification

• npx qunit test/highlights.decorations.tests.js → 5 passing
• npm test → 124 passing (119 baseline + 5 new), 0 failing
• npm run webpack → success

⚠️ Cursor is not installed in the dev environment used here (code CLI present, cursor absent), so final visual confirmation in Cursor should be done by a maintainer. The fix is justified by the confirmed upstream issue and the decoration-CSS reasoning above, and VS Code appearance is provably preserved (same border/radius/background values, only re-scoped).