Suggestion tooltip: find a backend for GNOME and KDE Wayland

[vstrelnikof]

The spelling-suggestion tooltip currently renders on wlroots compositors (Hyprland, Sway — via wlr-layer-shell) and X11 (override-redirect). On GNOME and KDE Wayland the popup crate is a no-op, so the feature silently degrades to the keyboard accept-chord with no visible list — near-useless until a backend exists. The engine side works everywhere; this is purely a "how do we draw an unfocused overlay" problem.

This is a research issue first: the deliverable is a written recommendation (or a prototype), not necessarily a full backend.

Directions worth evaluating:

• KDE: KWin implements the layer-shell protocol — verify which version/features it exposes to third-party apps and whether our anchor math (output-local coordinates, transforms) carries over. If it just works, KDE might be a small PR, and the "GNOME/KDE" gap statement splits in half.
• GNOME: no layer-shell for third-party apps, and no compositor-agnostic way to position an override surface. Candidates: a companion GNOME Shell extension the app talks to over D-Bus (precedent: several on-screen-display projects), or accepting an xdg-popup-shaped compromise, or declaring GNOME out of scope with a documented reason.
• Whatever the surface, it must never take keyboard focus (that's the whole point of the tooltip) and should reuse the existing anchor chain (AT-SPI caret → window bottom-centre → screen edge).

Where to look: crates/poltertype-popup/ (backend trait + wlroots/X11 implementations), docs/PLAN.md for the anchor-chain design.

[vstrelnikof]

Researched, and the answer changed the question. PR #17.

KDE was never broken. Before designing a backend for it, we ran the
existing one against an actual KWin — a nested kwin_wayland 6.7.3. The popup
crate selects linux-wayland-layer-shell and logs popup surface mapped: the
surface is created, configured by KWin and mapped, exactly as on Hyprland. KWin
has implemented zwlr_layer_shell_v1 for years and hands it to third-party
clients. No code was needed and none was written.

GNOME is not a no-op either. Mutter genuinely has no layer-shell
(mutter#973, open since
2019), but the factory's second probe is DISPLAY, which on a GNOME Wayland
session points at XWayland — and an override-redirect X11 window maps and
displays there. Verified in the shape that matters: forcing the Wayland probe
to fail inside a live Wayland session selects linux-x11-override-redirect and
logs popup window mapped.

So the real gap is neither GNOME nor KDE. It is a Wayland session with
neither layer-shell nor XWayland, which is rare and getting rarer.

The claim lived in five places — the crate docs, the factory, the Wayland
backend's module docs, the NoLayerShell error message a user would actually
see, and CLAUDE.md — and had been wrong since it was written. Worth naming
the shape of that mistake: the backend has always probed the compositor;
the prose was a hand-maintained table of desktop names. A probe stays right
when the world changes. A list of names goes stale silently.

What actually was missing on both, and is now fixed: the anchor. The Linux
focus tracker returned NoopFocusTracker for anything that is not Hyprland or
X11, so the tooltip had no window rect and no caret and fell back to
screen-bottom-centre. The rect is genuinely unavailable there. The caret is
not — AT-SPI is a session-bus service and does not care which compositor is
running; the watcher was simply never constructed, because it lived inside the
two branches that also had a window query. CaretOnlyFocusTracker now takes
that path: focused_exe() stays None, so nothing keyed off the focused app
starts guessing, while GNOME and KDE get the best anchor in the chain rather
than the worst.

Not attempted: a GNOME Shell extension over D-Bus. It would buy the window
rect — a worse anchor than the caret we now have — for a second distribution
channel, a review process, and a component that breaks on every GNOME release.
If Mutter ever ships layer-shell, the existing probe picks it up with no code
change.

Full write-up in docs/DECISIONS.md (2026-07-31).