swayfx-mosaic/docs/ARCHITECTURE.md

Architecture

This is the canonical technical contract for SwayFX Mosaic. The current implementation is an experimental single-row compositor layout. UX specifies user-visible behavior; known limitations identifies unvalidated boundaries.

Scope and authority

System	Responsibility
SwayFX Mosaic compositor	Runtime order, geometry, focus, camera, scene, input, transactions, IPC
User dotfiles	Session selection and personal key bindings
QuickShell	Future visualization and controls through compositor IPC; never a layout authority
Session V2	Future persistence/restoration helper; never a parallel runtime authority
SwayFX upstream	Base compositor and standard SwayFX behavior; maintained separately

The compositor is the runtime source of truth for Mosaic state. A consumer may request a change or save a snapshot, but must not maintain an independent authoritative window arrangement.

Runtime model and lifetime

The C model uses struct mosaic_workspace_state, struct mosaic_row, and struct mosaic_row_state (see include/sway/tree/mosaic.h). A workspace owns its Mosaic state and frees it with the workspace. The state owns row objects but never Sway containers. Row 0 borrows workspace->tiling, preserving the Sway tree's real ordered containers. Later model rows own their lists, not their children; they are not connected to runtime arrangement yet. A workspace also stores mosaic_previous_standard_layout separately from Sway's generic split-layout history.

flowchart TD
    W[Sway workspace and real tiling tree] --> M[Mosaic workspace state]
    M --> R[Row 0 borrows workspace tiling order]
    R --> G[Logical pending geometry]
    G --> T[Sway pending/current transaction]
    T --> S[Scene translation by scroll_x]
    S --> O[Output viewport and hit testing]

Distinct state

• Logical order: sequence in workspace->tiling for Row 0. Order is not inferred from screen coordinates.
• Row membership: only Row 0 is a runtime row today; multiple runtime rows are planned.
• Geometry: logical tiled rectangles in the workspace arrangement/transaction state. They may extend beyond the output width.
• Camera: row-local scroll_x, clamped to the arranged extent. It changes visibility without changing order.
• Focus: Sway's focus state, with a row-local remembered focused container. Focus can make an off-screen cell visible.
• Standard-layout history: workspace-local restore target for layout toggle mosaic.

Insertion after the active container and deletion reflow preserve contiguous logical order. Floating, fullscreen, and scratchpad retain Sway ownership and paths.

Standard layout compatibility

layout mosaic activates the native layout on one workspace. layout toggle mosaic enters Mosaic or restores that workspace's previous standard layout. Windows keep Sway node identity. When a standard tabbed or stacking layout wraps workspace windows in a root container, entering Mosaic promotes its children into Row 0; deeper groups remain one logical cell. Standard layouts remain usable in the same binary. The restore path is a layout transition, not process replacement.

Transactions and rendering

Sway's pending/current transaction model separates requested geometry from committed scene state. Mosaic calculates logical pending rectangles; scene positioning translates the tiling layer by -scroll_x. Camera updates and future animation must preserve agreement among committed scene positions, IPC geometry, and pointer hit testing. The current camera move is immediate; animated camera work is PLANNED.

The output renders a physical viewport of a wider logical row. Single-output headless tests demonstrate visual translation. A generic scene-tree clip at workspace/output boundaries is not yet implemented. Adjacent outputs may expose content outside a workspace's intended viewport; correct clipping and matching hit testing are a STEP20C concern. Do not infer multi-output safety from output-buffer cropping on one monitor.

STEP20C viewport design status

The STEP20C viewport proposal audits the actual SwayFX/SceneFX paths and recommends a PROPOSED Mosaic-owned viewport projection with matching input boundary. It is not implemented or accepted as a frozen contract. SceneFX 0.5 has no public generic scene-tree clip API. The isolated SceneFX clipping spike demonstrated a private inherited clip for tiled descendants and matching pointer hits, but a real XDG popup escaped the viewport and intercepted input on the next output. Multi-output Mosaic remains unsupported until all visual, interactive, damage and packaging boundaries are demonstrated together.

The proposal distinguishes logical row order and computed geometry from the workspace viewport, logical camera target, and any future presented camera position. During a transaction, scene and pointer input must refer to the same applied geometry/viewport generation. The current scroll_x is not copied into Sway's transaction state; a future implementation must define that boundary explicitly. These are design requirements for STEP20C, not claims about current behavior.

The isolated STEP20C feasibility probe concluded PARTIALLY FEASIBLE. A disposable public-API surface clip did not reliably constrain all rendered content on two horizontal headless outputs, and a partly visible client received pointer enter outside the Mosaic viewport. The probe did not qualify decorations, effects, saved buffers or popups. Approach C remains proposed; production STEP20C implementation and general multi-output support are not authorized by this evidence.

The follow-up SceneFX subtree spike is also PARTIALLY FEASIBLE. It confirms that a tree-local render/hit clip can cover ordinary tiled descendants in an isolated binary, including pointer pass-through to another output. The globally parented popup path violates that boundary. Its temporary static link demonstrates a possible package shape, not a decided SceneFX dependency or ABI contract. The next design must bind popups to the owner's viewport without changing ordinary floating, fullscreen or standard-layout popup behavior.

The later popup ownership probe demonstrated, in an isolated private build, a root-popup scene clip derived from the tiled owner’s applied Mosaic viewport. A tested nested popup inherited the clip; sampled visible popup input worked and the clipped-away popup did not intercept the ordinary client on the adjacent output. Standard splith behavior was restored after layout exit. Valid-serial XDG grabs and fully hidden popup owners were untested in that probe; additional output geometries, complete damage/effect coverage and package maintenance also remained unresolved. This evidence does not authorize production STEP20C or multi-output Mosaic support.

The valid-grab and hidden-owner probe measured a real button serial passed to xdg_popup.grab and compared unmodified SwayFX, private standard layout and private Mosaic. It found that the initial viewport-only popup clip leaked pixels and hits when the owner was entirely offscreen. A further disposable guard based on the owner's applied scene visibility suppressed the sampled leak while preserving the popup for camera return. The private guard does not establish a production transaction-generation contract, offscreen-popup UX policy or SceneFX ABI/package decision; broader STEP20C gates remain open.

The preserved source fixture reconstructs the private guard from pinned bases and patches. It supplies reproducible evidence, not a compositor-owned STEP20C implementation.

The subsequent nested valid-grab probe used two real pointer-button serials and a true toplevel → popup A → popup B XDG parent chain. In a disposable guarded binary, both popup levels were interactive within the horizontal two-output Mosaic viewport, absent from sampled clipped visual/input positions, and restored after a fully hidden owner returned without popup re-creation. The unmodified SwayFX and private standard-layout controls preserved ordinary sampled popup behavior. This narrows the feasibility gap for nested grabs but does not define a production transaction, damage, output-geometry or SceneFX ABI contract.

The vertical-output nested-grab probe then placed HEADLESS-2 below HEADLESS-1 and used a parameterized private virtual pointer. Unmodified SwayFX and private splith both rendered and received input on A/B popup portions actually crossing y=600; the guarded private Mosaic build omitted those sampled lower pixels and popup hits while preserving upper interaction. A hidden owner returned with the same mapped popups, and splith restored the sampled ordinary lower-output behavior. This is vertical popup evidence only. The production compositor still has no qualified cross-output viewport clip or committed scene/transaction contract.

The negative-origin nested-grab probe measured HEADLESS-1 at (-800,0) and HEADLESS-2 at (0,0) in both standard controls and private Mosaic. In the pinned wlroots path, the virtual-pointer absolute fraction maps through the signed output-layout box; the runner verified that global_x+800 reaches the requested output and client surface. The disposable applied SceneFX clip and popup-owner guard excluded sampled right-output A/B pixels and hits, while standard layouts retained real crossing and popup buttons. A fully hidden applied owner and return to splith also passed at measured points. This remains experimental evidence: production viewport clipping, transaction consistency, continuous damage coverage and SceneFX ABI ownership are not settled.

The later temporal damage probe kept that same negative-origin geometry and repeated genuine nested XDG open/close cycles, camera hide/return and outside dismissal. Sampled private Mosaic captures restored former popup regions without ghost pixels; the private standard layout repeated control behavior. Timed SceneFX damage extents and private SwayFX output-commit records covered changed pixels in sampled capture intervals, but neither the extents nor accepted commits identify all presented frames. The current Mosaic scroll_x still lacks a native transaction-generation contract binding applied scene, clip and hit testing. The temporal evidence is PARTIALLY_FEASIBLE, not a production rendering or ABI guarantee.

The presentation identity probe then observed native wlroots 0.20.2 headless output commit and deferred present callbacks in a separate private compositor. Each observed present paired with its accepted native (output, commit_seq), and a scene snapshot inside each commit callback recorded the applied tiled position and clip. Ten settled target transitions showed matching applied scene/clip values at 400 and 1280, while some faster command intervals had no commit. During initial layout setup, a presented commit exposed logical camera 1760 with applied scene and clip still at 1120; this reinforces the distinction between requested and applied state. The applied scene and clip themselves agreed. Headless present is an idle-delivered simulated event, and neither it nor sampled grim images identify the pixels of that exact output-state buffer. Pointer events remain a separate time-ordered stream. The result is PARTIALLY_FEASIBLE and does not define a production transaction or SceneFX ABI contract.

The subsequent private output buffer identity probe read sampled actual GBM output-state buffers through SceneFX FX texture readback, after an audited wlroots 0.20.2 buffer lock. In the fixed negative-origin headless matrix, 32 selected buffers matched their native commit callback, commit_seq and headless present; their sampled A/B pixels agreed with the applied tiled scene and clip at 400/1280. Three buffers were read again while locked at present with identical bytes. Four other selected locks were released before their later present events because a HEADLESS-2 repaint reached a global release in the private probe. This is an instrumentation scope defect, not a demonstrated replacement by a new HEADLESS-1 submission. The initial logical-1760/applied-1120 state also had an identified buffer and headless present, without a demonstrated pixel or hit defect. This does not create a production buffer API or transaction-generation contract. FX readback uses an internal GPU synchronization and measurably delays the commit path; frame-specific input consistency, other effects/renderers and physical presentation remain unproven. The result is PARTIALLY_FEASIBLE and multi-output Mosaic stays unsupported.

The applied state/input probe then corrected the private buffer hold to be output-scoped. A sampled HEADLESS-2 repaint between a retained HEADLESS-1 commit and its present did not release that reference. Three selected visible/hidden/returned buffers matched their applied scene/clip, native commits and headless presents, and were reread byte-identically while locked. A private experimental epoch marks some popup and transaction arrangement sites and appears in both build and executed hit-test traces. It is not a native atomic transaction generation and does not cover every render/input mutation. Client delivery under a real XDG grab was checked separately from the scene hit. This PARTIALLY_FEASIBLE result does not change production scene, transaction, input or SceneFX ABI contracts.

The mutation coverage probe audited compositor, private SceneFX and wlroots 0.20.2 mutation owners and ran a bounded trace-only headless matrix. Camera/clip and popup lifecycle observations agreed with scene hits and actual client events. A same-size client recolor changed pixels while applied geometry and the partial epoch stayed fixed; an empty surface input region changed the executed hit at the same point and epoch without changing sampled pixels. Thus asi_applied_epoch cannot identify all hit-relevant mutations. A native output (output, commit_seq) records a submission, not the later input state. Distinct geometry/hit, content and presentation identities are candidate research boundaries; whether existing event-loop ordering alone is enough across build, present and input remains UNDECIDED. No new production generation or transaction contract is accepted by this probe.

The independently replayed commit/present/input ordering probe confirms in source that wlroots headless queues present as an idle callback after an accepted output commit, while an applied Wayland input-region commit changes the region consulted by later scene hits. In five bounded cycles per mode, all 20 applied target input mutations in private standard and Mosaic occurred after the preceding matching present; none landed inside a native commit-to-present interval. Input-only hit and client delivery matched the effective region in sampled states. This leaves the cross-time Mosaic clipping invariant UNDECIDED and adds no production generation contract. A later hit may legitimately use newer input than a previously committed output buffer; identity of these states must remain distinct in further probes.

The follow-up private event-loop pending-input probe submitted ordinary client region requests without waiting for each camera or rendering IPC response. All 20 target mutations applied in two instrumented modes, but none occurred between a matching native headless output commit and present. This is INCONCLUSIVE for the unexercised temporal case; neither a Mosaic clipping violation nor the need for an additional applied generation was demonstrated. Client flush, server application, submitted buffer and later hit remain separate identities. The temporal line is closed for this backend/matrix after five attempts per mode, while the overall cross-time architectural question remains UNDECIDED.

STEP20C effects and damage evidence

The STEP20C effects/damage probe used a private inherited SceneFX clip with genuinely visible translucent blur and an ordinary shadow crossing the neighboring output. The sampled Mosaic blur stayed on its output; the ordinary 27,594-pixel shadow spill was absent there under Mosaic. Three settled hide/return pairs reproduced the original pixels, and sampled private damage covered changed pixels. The Mosaic interior shadow was not independently visible, while optimized blur cache correctness, out-of-viewport source sampling, popup effects and saved buffers remain unqualified. The private result is PARTIALLY_FEASIBLE and does not establish a production rendering or transaction contract. STEP20C remains NEXT; multi-output Mosaic remains NOT YET SUPPORTED.

IPC

On a Mosaic workspace, get_tree reports layout: "mosaic" and adds a mosaic object with active_row and rows. Each row entry contains ordered child node IDs, scroll_x, and focused node ID or null. Existing Sway fields retain their shapes; normal workspace/window events and get_workspaces paths remain in use. This is an extension to the Sway layout vocabulary: strict IPC consumers with a closed layout enum must recognize mosaic. Additional rows in the model are not a promise of usable multi-row IPC behavior.

Package and persistence boundary

The ordinary SwayFX package owns /usr/bin/sway; Mosaic owns the alternate /usr/bin/swayfx-mosaic. This is a pinned compositor build with a Mosaic patch, not a runtime plugin. Packaging specifies coexistence and file ownership.

Session V2 restoration is PLANNED. It must restore Mosaic activation, ordered rows, row membership, active row, camera positions, remembered focus, and stable window identities through compositor-owned state. It must not become another source of runtime truth.

Non-goals

• Replacing Sway workspaces or standard layouts.
• Replacing QuickShell or creating a new desktop shell.
• Simulating tiling with floating windows or a repositioning daemon.
• Embedding personal dotfiles or keyboard bindings in the package.
• Maintaining a permanently diverged SwayFX fork instead of a reviewable delta.

Effects freeze audit: the historical campaign exceeded its three-pair budget (15 pairs in five Mosaic runs); all evidence is retained. A separate fresh rebuild reproduced the main counts with one pair under the corrected campaign guard. The traced reference had a different effective size, so instrumentation neutrality and its cause remain unqualified. See the effects report. This adds no contract.

The preserved fractional scaling probe is PARTIALLY_FEASIBLE for static 1.25 uniform/mixed and 1.5 mixed headless outputs. Actual modes/PNGs and logical pointer motion were calibrated. Sampled root/nested popup and shadow spill disappeared on the neighboring output; visible blur, terminal popup columns, two settled camera returns and available physical damage were consistent. Opaque tiled-overflow controls, permitted interior shadow, restored Mosaic hit, caches and intermediate frames remain unqualified. Ten failed early calibrations and their source-provenance limit are retained. The two-pair global budget was respected. The next distinct gate should test resize/migration of owner/output clips; rotation, saved buffers and ABI/package remain separate. This is evidence only: STEP20C stays NEXT, production multi-output NOT YET SUPPORTED, STEP20D–G PLANNED.

The preserved dynamic output ownership probe is PARTIALLY_FEASIBLE: actual logical resize 800→960→800 and workspace migration H1→H2→H1 retain workspace/tiled identities and update sampled applied tiled/root-popup clips. Vacated outputs show exact background after stabilization. Two standard controls fail final reference calibration through fixture occlusion; Mosaic's corrected calibration passes. Maximum-scroll clamp, migrated clipped popup input, independent opaque tiled spill, effects and intermediate frames remain unqualified. Budgets are exhausted at six reconfigurations and six migrations, with zero additional camera pairs. No production change follows. Rotation is the next separate geometric gate; other open STEP20C gates persist. The freeze audit verified the original evidence manifest before an offline replay rewrote the temporary inventory; the report records that limitation. STEP20C stays NEXT, multi-output NOT YET SUPPORTED.

The rotated-output probe is PARTIALLY_FEASIBLE for sampled applied 90° geometry, logical PNG/pointer calibration, fully hidden owner guard, physical damage coverage and R0 return. The grim PNG is logical-oriented at 1000×750 while the R90 physical mode is 750×1000. Pristine R90 capture failed through a fixture IPC-direction error, and the Mosaic B popup never reached the boundary, so partial popup clipping under rotation remains UNDECIDED. No applied-geometry generation contract or production behavior is accepted by this evidence.

The separate static R90 partial-popup probe measured a real Mosaic B box crossing x=0 by 72 logical pixels, with B visible/clickable inside H1 and absent visually and from client delivery on H2. Its applied root clip and nested inheritance matched the H1 owner viewport in the sampled state; pristine and private-standard controls both drew and handled the crossed portion. Result PARTIALLY_FEASIBLE because the clipped-away part of A was not independently clicked and no atomic render/input generation was proved. The architecture remains experimental.

The local saved transaction buffer probe found saved_surface_tree under view->scene_tree as a sibling of live content_tree; SceneFX retains the source buffer at creation and a private trace observed saved render entries and normal removal after ACK/timeout. The experimental Mosaic tiled clip is an ancestor of the view tree, so a saved tree has a plausible inherited path. The measured target was outside H1 after the applied camera reached 1760, preventing a boundary-clipping conclusion. This remains INCONCLUSIVE, with no production architecture change.

The separate target-crossing saved-buffer probe then measured the actual source and saved scene buffer crossing x=0 after a single calculated camera correction. Its target saved node rendered with a visible region ending at x=0 and retained target pixels on H1; no target SHM color appeared on H2. A 253-pixel whole-H2 reference difference and incomplete Mosaic timeout/input/damage/lifetime evidence limit the result to PARTIALLY_FEASIBLE and architecture UNDECIDED. No production contract changes. Offline freeze analysis strongly supports a blinking Kitty block cursor for the repeated H2 reference difference, without proving its cause. E2's x=-30 visible bound matches the applied tiled clip narrowed by gaps; it is not evidence of a distinct content clip. Target timeout cleanup remains open.

The separate natural timeout and input gate uses a static SHM H2 reference and one Mosaic transaction. The actual target source and saved node both crossed x=0; the saved node rendered only its H1 portion and its tree was removed after a real 2500 ms timeout with serial 50 held and the client alive. The whole H2 RGB image was identical across two preflight, held and applied captures. Two sampled pointer positions found the H2 reference surface and the visible H1 saved node respectively; the latter is not a Wayland surface. This closes the sampled saved-target ACK and timeout clipping matrix, PARTIALLY_FEASIBLE, without proving full damage, all input paths or native GPU lifetime. The earlier crossing campaign's own architectural verdict remains UNDECIDED. The new limited-matrix conclusion does not authorize a production implementation. Optimized blur/cache and SceneFX ABI/package remain separate gates.

The preserved optimized blur cache probe is PARTIALLY_FEASIBLE, architecture UNDECIDED. A real background layer under blur_tree triggered natural output-specific cache reconstruction and fresh central A/B/A filtered pixels in private splith and Mosaic. Sampled final tiled drawing excluded H2, whose whole RGB remained identical; output damage extents covered stabilized differences and sampled cache/FBO teardown completed. Shared boundary differences up to six RGB levels remain unresolved. Pristine C0 failed before captures through a duplicate configure fixture; its second initial image was reserved only after observation. No numerical budget was exceeded or run repeated. Instrumentation neutrality, whole-cache equivalence and intermediate frames remain unqualified. A separate pristine/untraced boundary-sampling gate should precede ABI/package qualification. This evidence freeze changes no production or UX contract. STEP20C stays NEXT, multi-output NOT YET SUPPORTED.

The preserved optimized edge causality gate is PARTIALLY_FEASIBLE, architecture UNDECIDED. Four completed fixed negative-origin runs qualify pristine/private splith and same-binary Mosaic OPT-off/on controls. Pristine/private full images and Mosaic trace-off/on full images are exact at six homologous states; common glass across layouts is also exact. Historical right/bottom differences persist without new traces, with max RGB delta 6, fresh central pixels and stable whole H2. Native FBO history causality remains unproved despite source/model support. R3 damage extents cover stabilized differences only. No clipping regression or required invariant violation is established. A separate SceneFX ABI/package audit is now justified, retaining this shared rendering limitation; optional selective GPU observation requires a new scoped ticket. This evidence is preserved without a production change. STEP20C stays NEXT, production multi-output NOT YET SUPPORTED.

The preserved SceneFX ABI/package audit is PARTIALLY_FEASIBLE, architecture SCENEFX_ABI_PACKAGE_REQUIRES_ARCHITECTURAL_DECISION. The unchanged current package builds with stock SceneFX and stages exactly /usr/bin/swayfx-mosaic. The shared experimental binary needs two clip exports absent from stock despite the same SONAME. Static embedding builds and resolves without a SceneFX shared dependency, but shared wlroots interposition, updates, security maintenance and license notices remain gates. A distinct private shared library would require a new file-ownership decision; upstream API acceptance is unknown. No integration or packaging contract is approved. The historical audit is frozen as evidence; STEP20C stays NEXT, production multi-output NOT YET SUPPORTED.

The preserved static distribution contract dossier is EXPERIMENTAL / PARTIALLY_FEASIBLE, conclusion STATIC_DISTRIBUTION_REQUIRES_CONTRACT_DECISION. One private Meson DESTDIR stage of the hash-qualified static executable contains exactly /usr/bin/swayfx-mosaic; this proves file shape only. The 85 overlapping exports and 31/34 wlroots scene bindings into the executable remain explicit paired-version constraints, not automatically defects. Only the pinned SceneFX 0.5 / observed wlroots 0.20.2 pair has non-graphical evidence. Notices (including generated protocols), security ownership, upgrades, rollback and graphical semantics remain gates. A disposable notice-section extraction passes mechanically; legal distribution compliance is not approved. The upstream API dossier is preserved and not submitted. Production strategy: NONE — USER DECISION REQUIRED. Packaging/UX contracts are unchanged; STEP20C stays NEXT and production multi-output NOT YET SUPPORTED.

Approved private STEP20C static candidate — 2026-10-13

The user approved four decisions for documentation and private prototypes only: private static SceneFX; separate complete notices under /usr/share/licenses/swayfx-mosaic/; project responsibility for embedded SceneFX maintenance; and qualification of each exact SceneFX/wlroots pair. See the decision register and bounded runtime gate. The current PKGBUILD, system SceneFX dependency, one-binary package, production sources and cumulative patch remain unchanged. The future private stage may contain only /usr/bin/swayfx-mosaic and justified regular notice files below the license directory. No private shared library, session, service, config or Sway symlink is allowed. No real installation is authorized.

Mosaic owns monitoring, impact review, rebuild and requalification of embedded SceneFX fixes, including renderer/shaders/dependencies/ABI/buffer lifetime/clip/ damage changes. Before public distribution, a release maintainer and security reviewer must be designated; no names or SLA are assigned. Preserve exact source, patch, compiler/options/dependency, notice, test and artifact manifests. A new relevant dependency build requires header/symbol/shared-layout inspection, rebuild, relocation/model/staging/headless checks and graphical requalification of relevant paths, then a separate publication decision. No automatic update subsystem is added. Ordinary SwayFX upgrades remain independent and standard SwayFX remains the fallback. All other pairs are NOT QUALIFIED.

These decisions do not settle popup UX or close STEP20C production gates. STEP20A DONE; STEP20B EXPERIMENTAL; STEP20C NEXT; STEP20D–G PLANNED. Production multi-output remains NOT YET SUPPORTED.

The private static runtime gate is EXPERIMENTAL / PARTIALLY_FEASIBLE, architecture UNDECIDED. Real /usr static build/model/stage/notices/loaded-library identity/headless startup pass. Sampled H1 target pixels survive with zero target colors on H2, but whole-H2 reference equality fails through a focus/decor mismatch; input and splith return are NOT TESTED after the single run stopped. No rerun or algorithm modification occurred. Distribution notice closure, named release/security roles and broader ABI/graphics gates remain open. STEP20C NEXT, multi-output NOT YET SUPPORTED.

The static focus/input/standard follow-up is EXPERIMENTAL / INCONCLUSIVE, architecture UNDECIDED. One run reused the exact static binary, verified loaded wlroots/output geometry, calculated camera and observed the applied clip. A fixture incorrectly required stored current_border_width=0 under border none; actual IPC retained thickness 2 with empty decoration and full-size client rectangles. The run stopped before any PNG, click or splith transition. Those gates are NOT TESTED; no clipping defect or need for an algorithm change is demonstrated. No retry/build/production change occurred. A separately authorized corrected-fixture gate is required. STEP20C stays NEXT; production multi-output remains NOT YET SUPPORTED.