swayfx-mosaic/docs/design/STEP20C_NEGATIVE_ORIGIN_NESTED_GRAB_PROBE.md

STEP20C negative-origin nested-grab probe

Status: EXPERIMENTAL / FEASIBLE for the measured negative-origin matrix. This is a disposable feasibility test, not a production clipping change or UX decision. The canonical architecture, UX, limitations and roadmap retain authority.

Problem and hypotheses (recorded before fixture adaptation)

An output at global x=-800 makes global scene and pointer coordinates signed. The virtual-pointer protocol instead transmits unsigned absolute coordinates with unsigned extents. Passing a negative global X directly to that protocol would wrap it. Hypothesis H1: with two adjacent 800×600 outputs, wlroots maps normalized pointer X through the layout bounding box; therefore injected_x=global_x-min_x=global_x+800 with extent 1600 reaches the requested global point. This must be confirmed from the pinned wlroots path and from client pointer events on both outputs before any clipping result is valid. A protocol-level injection acknowledgement alone is insufficient.

Hypothesis H2: the disposable SceneFX subtree clip, owner-derived popup clip and hidden-owner guard use signed applied scene positions correctly when the viewport begins at -800. The control layouts must first demonstrate real pixels and button delivery beyond x=0; configured rectangles alone do not prove a crossing. Hypothesis H3: changing Mosaic back to splith removes the private clip while both XDG popups remain mapped.

Coordinate contract and requested geometry

Quantity	HEADLESS-1	HEADLESS-2
Requested global origin	(-800,0)	(0,0)
Requested size, scale, transform	800×600, 1, normal	800×600, 1, normal
Global X interval	[-800,0)	[0,800)
Local X	global_x+800	global_x

The requested layout bounds are [-800,800) × [0,600), with border x=0. get_outputs is authoritative. If the compositor normalizes these origins, the negative-origin test is INCONCLUSIVE and the actual rectangles must still be retained. Workspace 1 and the tiled popup owner belong to HEADLESS-1; workspace 2 holds wev on HEADLESS-2.

For a requested global point (-100,190), HEADLESS-1 local X is 700 and candidate virtual-pointer coordinates are (700,190) with extent 1600×600. For (100,190), HEADLESS-2 local X is 100 and the candidate transmitted X is 900. The runner must log requested signed global point, layout minimum, transmitted normalized point, extents, reached output, client-local motion and button events. It must reject out-of-bounds and nonintegral injection values.

Mosaic row order and logical owner IPC rectangle remain distinct from camera and applied scene placement. For each sample, record logical rect, window_rect, scroll_x, applied owner/popup scene positions, SceneFX clip, XDG role configure rectangles and actual captures. The candidate applied content X is owner.rect.x - scroll_x + owner.window_rect.x in Mosaic; the compositor scene log and captured pixels must verify it. Popup A's XDG configured rectangle is relative to the toplevel surface; B's is relative to A's surface. Their resulting global rectangles are estimates until scene position and pixels corroborate them. XDG anchor rectangles must lie within their effective parent window geometry.

Experimental method and acceptance

Reuse the historical nested client, the vertical fixture's parameterized pointer and the same three pinned private SceneFX/Mosaic patches, including the hidden-owner guard. Adapt only the new fixture directory. Run identical clients in (A) unmodified SwayFX splith, (B) private compositor splith, and (C) private compositor mosaic, all with verified negative output origins. Use isolated runtimes and sockets, swaymsg -s, two-output captures, IPC, WAYLAND_DEBUG=1, compositor logs and structured measurements. A separate positive-origin horizontal subset checks regression. No installed library, daily session or production source is touched.

1. Validate signed pointer targeting on both outputs, including round trip and left/right buttons. Failed targeting invalidates later graphics tests.
2. Open two visible popups: TOPLEVEL button serial grabs A, then A button serial grabs B; verify distinct actual serials, parent XDG identities, configure/ack/map order and interaction with each surface.
3. Test B crossing x=0, then A and B crossing. Both standard controls must show actual popup pixels and B/A button targets on HEADLESS-2. Mosaic must preserve visible left portions and suppress right popup pixels/hits, with ordinary-client events interpreted under normal XDG grab semantics.
4. In separate runs, test outside-click dismissal, fully hidden applied owner with no click and with a click, and layout splith while popups remain open.
5. Replay a horizontal positive-origin subset with the same adapted clients and injector. Derive samples from actual configured/scene geometry and capture data, never from assumed historic positions.

Each scenario retains output JSON, signed coordinate conversions, pointer events, owner IPC/Mosaic state, XDG configuration and identities, scene clips, two captures per stage, lifecycle events, process exit codes and a measurements.json. Rendering, hit testing, XDG grab, coordinate transform and lifecycle verdicts are separate. A screenshot alone or a protocol send alone cannot establish a pass. Keep failures and inconclusive runs visible.

The probe is FEASIBLE only if negative origins and control crossing are real, Mosaic excludes measured clipped pixels/hits while preserving visible interaction, and private standard layout has no blocking regression. It is PARTIALLY_FEASIBLE for demonstrated core behavior with matrix gaps, BLOCKED for reproducible rendering/input leak, XDG violation or blocking standard regression, and INCONCLUSIVE if geometry, crossing or pointer events cannot be established. These labels apply only to this probe.

Initial observations and correction history

The first unmodified SwayFX pointer run recorded get_outputs with active HEADLESS-1 (-800,0,800,600) and HEADLESS-2 (0,0,800,600), both scale 1 and transform normal. Thus SwayFX did apply the requested negative origin. The fixture transmitted 302 for global -498 and 830 for global 30. The owner received wl_pointer.enter with surface-local X 100, then 105 on the return trip; wev received enters with local X 28, then 33 (its surface starts inside the container). This establishes preliminary two-output targeting, subject to full button validation. Evidence is retained in run-negative-control-pointer-initial/ under the new laboratory.

The first calibration checker falsely required POINTER_MOTION lines. An inter-output move produces wl_pointer.enter at the destination instead; both clients logged those enters. The checker now accepts enter or motion. That first run exited with a fixture RuntimeError after its captures; it is not classified as a compositor or clipping failure.

The actual wlroots 0.20.2 path confirms H1 at the source level: types/wlr_virtual_pointer_v1.c forms normalized x / x_extent, while types/wlr_cursor.c converts that fraction to mapping.width * x + mapping.x using the output layout box when no device mapping overrides it. Sway's absolute-motion path uses the wlroots cursor. With box X -800, width 1600, and transmitted X global_x+800, the nominal reconstructed global point is global_x. Client events remain the empirical check for device mapping and targeting.

The completed pointer runs in unmodified SwayFX, private splith, and private Mosaic each reached both outputs on two round trips. TOPLEVEL received right and left press/release (button=273/272); wev received left and right press/release. In the control, requested global (-498,150) reached TOPLEVEL local (100,125) and requested (30,80) reached wev local (28,55); the approximately 2 px X and 25 px Y differences are the client surface inset, not an output-origin shift. Private splith repeated those coordinates, and private Mosaic used global (-458,150) for TOPLEVEL local (100,125) after its camera translation. All protocol X values were nonnegative and inside the 1600×600 extent.

All three negative-origin visible runs mapped A and B and kept them open through separate right clicks. A's role configure was (50,80,250,170) and B's (20,60,170,130) relative to its parent. Popup pixels were confined to HEADLESS-1 in this reference placement; HEADLESS-2 contained zero exact A yellow or B green pixels.

The negative-origin b_cross controls both configured B at (-178,165,300,130) after applied-owner translation and measured 15,860 exact B-green pixels on HEADLESS-2 at the open capture. Private Mosaic configured B at (-138,165,300,130) and measured 0 B-green pixels on HEADLESS-2 while retaining 17,940 on HEADLESS-1. These are preliminary render measurements; button traces and the remaining matrix are under review.

At the B crossing point, both standard controls delivered two B right-button events and retained 15,860 B-green pixels beyond x=0; private Mosaic delivered the left visible B button, then delivered the right-output button to wev, with no right-output B button. Its legitimate outside click then caused popup_done B before A. In the both_cross placement, both standard controls had 6,120 A-yellow and 27,560 B-green pixels on HEADLESS-2; private Mosaic had 0 of each there, while retaining 30,080 A-yellow and 6,240 B-green pixels on HEADLESS-1. A separate a_cross run is used for A's right-output button, since a first outside click can dismiss the grabbed hierarchy.

Comparative results

All three compositors used HEADLESS-1 (-800,0,800,600) and HEADLESS-2 (0,0,800,600) at scale 1, transform normal. The same nested client used TOPLEVEL XDG object 9, A object 12 and B object 15 in each run; A named TOPLEVEL as parent, B named A. In b_cross, both standard controls took real TOPLEVEL left-button serial 69 to grab A and A left-button serial 74 to grab B. Private Mosaic used 63 and 68. Each GRAB_REQUEST used the exact target event's serial; configure, xdg_surface ack and MAP followed in order. No protocol error or immediate grab denial occurred. The anchored rectangles stay within their effective parent geometry: the horizontal b_cross A anchor X 200 is inside the standard top buffer width 420, and B anchor X 220 is inside A's width 300; both_cross A anchor X 350 is likewise inside 420, B anchor X 160 inside A's width 350.

Scenario	Unmodified SwayFX splith	Private splith	Private Mosaic	Measured distinction
Pointer reference	PASS	PASS	PASS	Signed global points reached both clients twice; left/right press and release on both outputs.
A/B fully visible	PASS	PASS	PASS	Two grabs, mapped distinct surfaces, A/B right clicks, no unexpected dismissal.
B crosses x=0	PASS	PASS	PASS	Standard B right pixels 15,860 and B button; Mosaic right pixels 0, B left button, wev right button and normal outside dismissal.
A and B cross x=0	PASS	PASS	PASS	Standard right A/B pixels 6,120/27,560; Mosaic 0/0 with separate visible A/B clicks.
A crossing hit in separate run	PASS	PASS	PASS	Standard right A button; Mosaic no right A button, wev button and normal dismissal.
Outside click and lifecycle	PASS	PASS	PASS	popup_done B then A; client destroys B then A; ordinary client receives left button.
Owner fully hidden, no click	N/A	N/A	PASS	Applied scene owner (-1440,0,640,600) does not intersect viewport (-800,0,800,600); A/B pixels 0 then return with same XDG IDs.
Owner hidden with click	N/A	N/A	PASS	No hidden A/B button; B then A dismissed; pixels stay absent after camera return.
Mosaic → splith with A/B open	N/A	N/A	PASS	Right B pixels return to 15,860; B receives right button at (20,195) before any popup_done.
Positive horizontal regression	PASS	PASS	PASS	Standard B right pixels 15,860; Mosaic 0 with the same button-routing distinction as the historical horizontal probe.

The B-crossing point was global (20,195) on HEADLESS-2. In standard controls its open-capture RGB was B green (32,208,160) and its right click went to B. In private Mosaic that point was ordinary background (238,238,238), and its right click went to wev; B received no button there. The corresponding Mosaic visible B point (-98,195) was green and delivered a B right button. For A, the separate standard crossing point (87,125) was A yellow (224,192,32) and delivered an A button; Mosaic's scene-adjusted point (127,125) was ordinary background and delivered no A button, while visible A/B points remained interactive. Ordinary-client delivery and popup dismissal are distinct observations; dismissal alone is never counted as a clipped-surface hit.

The private PROBE_POPUP_CLIP log recorded clip local (-242,-25,800,600) in the sampled Mosaic runs. Relative to viewport origin (-800,0), this gives applied root popup-tree scene origin (-558,25); combining the logged XDG configure offsets gives A (-358,105,300,170) and B (-138,165,300,130) in b_cross. These match the independently calculated owner/window/camera positions and the captured crossing colors. In both_cross, A is (-208,105,350,170) and B (-48,165,300,130). A negative global origin is therefore present in both the applied clip and popup surface geometry, not only in an IPC label.

For hidden owner, the compositor logged PROBE_POPUP_HIDDEN_OWNER with applied (-1440,0,640,600) versus viewport (-800,0,800,600), proving empty intersection from applied scene state. The no-click run's left A/B exact pixel counts were 42,200/17,940 → 0/0 → 42,200/17,940. There was no popup_done before return and no new XDG CREATE or MAP; XDG IDs remained A 12 and B 15. The IPC owner stayed container ID 8 and focused=true in open, hidden, hidden-hover and returned snapshots; its logical rectangle stayed (-160,25,640,575) while the applied scene position changed with camera. In the separate hidden-click run, neither popup received the right button; B then A received popup_done, were destroyed child first and did not return. With both still open, switching to splith restored the standard right B-green count 15,860 and a B right-button press/release with no earlier dismissal.

The positive-origin regression used HEADLESS-1 (0,0) and HEADLESS-2 (800,0) with the same compiled clients and pointer. Both standard controls again produced 15,860 B-green pixels on HEADLESS-2 and delivered the right-output button to B. Private Mosaic produced 0 and delivered the button to wev, followed by normal B/A dismissal. Counts happen to match the historical horizontal probe after checking the actual applied placement; the test did not assume their equality.

Scope of conclusion

This negative-origin, two-output, nested-valid-grab experiment is FEASIBLE for the measured placements and event points. It establishes correct signed-to-absolute pointer conversion in the observed wlroots path, real control crossing, sampled private clipping/hit behavior, hidden-owner guard behavior and standard-layout restoration. It does not prove continuous pixel coverage between samples or define universal offscreen-popup dismissal UX. Scaling, rotation, output migration/resize, complete damage/effects, transaction generations and ABI/package maintenance remain NOT TESTED. STEP20C remains NEXT and production adjacent-output clipping remains NOT YET SUPPORTED.

Independent freeze replay (2026-10-11)

A fresh laboratory, /tmp/swayfx-mosaic-nested-grab-1DzZQbcH/, was built by the preserved prepare.sh from full Git checkouts, rather than an archive. Its build log is /tmp/mno-prepare-ltr6sSlW. Fourteen new run directories cover pointer targeting on both outputs in all three configurations, negative-origin B crossing and A+B crossing in all three, private Mosaic hidden-owner return and splith return, and positive-origin B crossing in all three. analyze.py decoded the new PNGs. The fresh output JSON again reported (-800,0,800,600) and (0,0,800,600) for negative-origin runs. In each pointer run, the owner received destination wl_pointer.enter at local (100,125) and (105,135), and wev received enters at (28,55) and (33,60); both clients also logged press/release events. These client events, not injector acknowledgements, validate signed-global targeting.

New control and private-standard B-crossing captures each contain 15,860 B-green pixels on HEADLESS-2 and two B button presses. Private Mosaic again contains zero B-green pixels there, preserves 17,940 on HEADLESS-1, delivers the visible B press, and sends the clipped-side press to wev, followed by normal B/A popup_done. For A+B crossing, both standard controls again contain 6,120 A-yellow and 27,560 B-green right-output pixels; private Mosaic contains zero of either there while retaining 30,080/6,240 on the left. The fresh logs show distinct A/B XDG objects and actual A/B grab serials (69/74 in the control B run; 63/68 in private Mosaic), with matching button serial, configure/ack/map order and no recorded protocol error. Serials varied in another control run, as expected; their numeric values are run evidence, not a protocol constant.

In the new no-click hidden-owner run, scroll_x moved 400 → 1280 → 400, both popup colors changed 42,200/17,940 → 0/0 → 42,200/17,940 on HEADLESS-1, the same XDG objects remained mapped, and no popup_done preceded return. In the new layout-return run, splith restored 15,860 B-green pixels on HEADLESS-2 and a B press without prior dismissal. The positive-origin B-crossing subset reproduced the same control/private distinction. These are sampled stabilized observations: the replay did not capture every presented frame or establish complete damage coverage.

Reproduction, provenance and validation

The retained laboratory is /tmp/swayfx-mosaic-nested-grab-NWy4A23x/; its preparation log is /tmp/mno-prepare-4FC2ZHqF. The source-only fixture documents a fresh replay from a full Git checkout. The three unchanged private patches applied with git apply --check to Mosaic cd550382b311efdbfe3ed892f4e1d281d852e754 and SceneFX 37ccd723bef49e6891156ffafce8f549f01446cc. The historical pointer extents patch alone adapted a disposable copy of pointer-daemon.c; no new private patch was needed. The historical nested client was byte-identical. The environment used /usr/bin/sway 0.6, wlroots 0.20.2, SceneFX 0.5.0 and Wayland client 1.26.0. ldd resolved the private Mosaic executable to the laboratory libscenefx-0.5.so and system libwlroots-0.20.so.

artifacts-sha256.txt lists 895 run files from the final and retained intermediate cases; its SHA-256 is 0f3d74cb3477aa24b5e0f6bcf84f736ffcd31280de04b08b4622a761046eb793. sha256sum -c verified 895/895. negative-origin-source-sha256.txt lists six source inputs; its SHA-256 is 8cbf6228c0bd317442262265692f4027e41fd82c2e7c7cd22bbb0e03ddaf729d, and sha256sum -c verified 6/6. Each run directory retains verified output JSON, signed injections, observed client-local pointer reference, XDG and WAYLAND_DEBUG=1 traces, IPC trees, scene clips, two-output PNGs, RGB/count analysis, popup_done order and process exit status. The intermediate failed pointer-calibration check and its evidence remain under run-negative-control-pointer-initial; earlier both_cross and toggle attempts remain under *-initial. The final both_cross runs were replayed with the settled runner. The first toggle sampled B on the left after return; the final run explicitly sampled its right-output portion.

Private SceneFX and Mosaic builds passed; the Mosaic Meson model test passed 1/1. C17 syntax checks passed for both client sources against generated protocol headers. Shell syntax, Python AST parsing without bytecode cache, document links, git diff --check, source hashes and artifact hashes passed. An independent structured matrix check passed pointer targeting, both grabs, crossing colors/buttons, outside dismissal, hidden-owner, layout return and positive regression. All completed case directories include actual exit codes; normal runner teardown produced compositor/client 0 and expected SIGTERM -15 for the persistent pointer and wev. Their private runtime directories were removed after reaping; no probe process remained. These validation passes support only the measured scientific conclusion above.

No production Mosaic source, production patch, PKGBUILD, installed library, historical fixture, UX policy or active session was changed; at the time of the original experiment nothing had been committed or pushed. The smallest next bounded experiment is repeated open/close and camera movement at this same negative-origin geometry, with frame-by-frame stale-pixel and damage observations. It would test a currently open gate without changing output transform or popup protocol.