[hetaoBackend]

Change

Addresses #216. Enable the fullscreen transcript scrollbar, including track-click jumps and thumb dragging. Reserve a three-column gutter so the larger hit target cannot swallow adjacent content clicks or text selection; narrow terminals retain at least one content column. An overlay cancels an active scrollbar drag on the next mouse event.

Adapted from drowzeys/minimax-code@0b24a7778741fda335a9a2074464c69911feb1aa, submitted under the existing MIT license in #216. Preserve drowzeys as co-author and record the adaptation and Engine file hashes in LOCAL_CHANGES L035. The original unrestricted three-column hit span failed the added content-click, text-selection and overlay-drag regressions.

Validation

• PASS: focused scrollbar and Engine local-delta suites, 25 tests. Real SGR input through VirtualTerminal covers track presses, thumb dragging, release, wheel events, widths 1/2/3/4/8, adjacent content mouse routing and selection, overlays, and the actual fullscreen ChatLayout.
• PASS: standalone typecheck, diff check, and L035 recorded-hash verification.
• PASS: pnpm verify full profile, all 14 gates on clean commit 70bda1ca11f4ed2e7594b1c8073ab7fb659f7839 (macOS arm64, Node 26.4.0), including source export, release tools, build, artifact, capabilities, status contract, CLI smoke, offline BYOK, policy and sandbox.
• NOT RUN: manual physical-terminal mouse acceptance, local Linux/Windows runs, and live services. VirtualTerminal coverage is offline; CI results are reported separately.

Publication and contribution checks

• Imported material, license permission, provenance and author attribution are identified; existing notices are preserved.
• No credentials, account data, real user content, internal source history or private review material is included.
• The new test was reviewed before regenerating release/public-source.json and registered in test/vitest-suites.json.
• Engine behavior and validation are documented in L035. Shared README content is unchanged; offline checks are not live-service acceptance.

Maintainer handoff

Publication scope or license changes: none.

Shared-source port: pending after public review.

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