Request PRIMARY for bounded middle-click paste #655

Active implementation checkpoint — 19 September 2026

Exact upstream heads are WebScene 91fa294f5616f94ba5a24b3b08cdf7307166fc6e and AppScene fe14e7b775d347693d6526b7c4d49c0dca082e52; unchanged Code OSS is 645f29cc3176500b4b5762ba887cf2a7f0ffdf2c. Local vscode-demo is c543d2ca9ff97f3c9e256c5eda4193fad5b53c13 and remains intentionally unpushed with Actions disabled.

Merged Linux exact-device work remains complete through WebScene #654 and AppScene #270. Linux CLIPBOARD/PRIMARY and inbound/outbound Xdnd are merged through WebScene #658 and AppScene #280. CSS animation direction #659/#660 and fill modes #661/#662 are merged. The current-main native compiler failure exposed by #662 is tracked and fixed by #663/#664; generated selectors now preserve compiled namespace-aware compound and attribute data. #666/#667 binds the remaining native test consumer to the namespace-aware compiler API, and #668/#669 restores complete structured selector parity comparisons. #665/#670 implements demand-free animation pause/resume, and #671/#673 enforces parser package size with both ratio and absolute byte budgets, and #672/#678 implements bounded eight-track animation-list coordination; #674/#681 implements zero-time/count semantics, #679/#680/#682 updates the native stale-track contract for inactive authored list slots, and #675/#683 adds bounded translate/scale keyframe sampling and retained paint composition. #684/#685 refreshes the generated Code OSS ledger and exact property identity count exposed by the post-merge main jobs. #676/#686 adds bounded retained ::before/::after animation lifecycle, scoped paint, owner events, cleanup, and demand-free settled state. Full-V8 package portability epic #687 is implemented by #688/#691 for GCC, #689/#692 for MSVC enum/ABI boundaries, #690/#693 for bounded embedded source literals, #694/#695 for const cascade roots, and #696/#697 for native test syntax. #677/#698 adds bounded retained WAAPI with document/element caps, lifecycle settlement, and demand-free inactive states. #699/#700 restores the 384-byte native document footprint by moving WAAPI-only maps and event state behind one lazy auxiliary allocation. #476/#702 adds bounded asynchronous document fetch and stale-generation suppression for dynamically inserted CSS imports, complementing #480/#484 authored-import reflection. #701/#703 adds browser-shaped Worker options, inherited credential policy, final response/CORS enforcement, generation-safe failure, and bounded slot retirement. #704/#705 adds an optional bounded redirect ledger and per-hop Worker status/scheme/loop/downgrade/origin/CORS/credential checks. #706/#707 extracts bounded request-header ABI v5 onto current main; AppScene #281/#282 and vscode-demo #10 populate the ledger in their host providers.

One-agent mode is enforced. The sole implementation agent is implementing bounded interactive reportValidity feedback under #708; no other implementation agent is active. Static fill does not sustain frame demand, and new animation work must preserve that bound without per-frame cascade, selector, or visual-tree churn.

Consolidations remain open and unmerged: WebScene #76 at b1f4977691938e636c1e16ac1c2f37c4d2128f4c, AppScene #65 at 0c018306458050a66a19d2c84abd3cf65a20832c, and vscode-demo #1. WebScene #662 and #681 portable V8 contracts passed. Focused #682 native document and Linux SDK runs 35448768204 and 35448768281 passed; #683 portable V8 contracts passed. The failed #683 current-main generated-ledger/property-count gates are repaired by #684/#685, and Linux CI job 105914816775 passed. Full-V8 Linux/Windows RID portability repairs are merged through #691-#697; current-main CI/NuGet runs 35454910272 and 35454910266 are retained; superseded PR, previous-main, and consolidation runs were cancellation-requested. AppScene #282 current-main companion run 35454940645 passed.

Ordered next work: #708 interactive validity feedback, then current-main CI/package triage and the remaining focused CSS and acceptance backlog, then physical Linux pixels, synchronization/device-loss/resize/soak/performance, VM/computer-use, and unchanged Code OSS Release qualification. Rich clipboard MIME, MULTIPLE/SAVE_TARGETS, and the recorded desktop-service/cross-platform backlog remain open.

Parent: #227. Related input/clipboard work: #86, #100, #331, #405, #440, #443 and #531. AppScene consumer: SceneTech/AppScene#273.

Problem

WebScene's typed clipboard host ABI only distinguishes ordinary clipboard reads and writes. On X11, middle-button paste must read the PRIMARY selection while Ctrl+V and navigator.clipboard continue to use CLIPBOARD. AppScene #271/#272 now implements both native selections, but WebScene cannot request PRIMARY.

Scope

• Add an ABI-compatible v1 clipboard-read flag such as WEBSCENE_HOST_REQUEST_CLIPBOARD_PRIMARY_V1; existing zero flags retain CLIPBOARD behavior.
• Admit the flag only for well-formed text/plain clipboard read requests and reject unknown combinations before host dispatch.
• Observe the existing Button2 pointer/mouse/auxclick sequence, preserve anchor navigation and normal event ordering, and request PRIMARY once after an uncancelled middle gesture.
• Treat mouseup or auxclick cancellation as a veto; settle the exact initiation point in implementation so one physical gesture produces at most one request.
• Apply PRIMARY paste only when the gesture focuses or targets a writable text control, including unchanged Code's hidden textarea after mousedown cursor placement; the current runtime does not claim contenteditable editing. Capture that control/context and dispatch the trusted paste flow only if it and the document generation remain live when the asynchronous result returns.
• Preserve ordinary navigator.clipboard, Ctrl+V and host clipboard requests as CLIPBOARD.
• Reuse the existing 16 completion-bearing clipboard-operation limit, 16 MiB payload limit, 1,024 aggregate host queue, five-second activation window, navigation/shutdown cancellation and late-completion rejection.
• No polling, unbounded queue, X11 dependency, browser/Electron/CEF/WebView integration, or VS Code-specific patch.
• Do not claim automatic PRIMARY publication; that requires a separate product-neutral selected-text provider signal.

Acceptance

• One uncancelled Button2 gesture on a focused writable text control issues exactly one flagged text/plain read and dispatches the trusted paste path at the current cursor.
• preventDefault, detached/stale targets, navigation, shutdown, timeout, malformed flags and saturated queues cancel or reject once without paste.
• An uncancelled HTTP(S) anchor keeps Preserve nested anchor activation and external-link disposition #531 external-new-context handoff; PRIMARY paste does not replace the anchor default action.
• Ctrl+V and navigator.clipboard remain byte-for-byte compatible with CLIPBOARD.
• Portable event/host contracts cover ordering, cancellation, stale completion, caps and a bounded repeated-gesture performance case.
• Packaged unchanged Code OSS middle-click paste remains an explicit downstream AppScene/Linux acceptance gate.