[19.0][ADD] web_timeline_gantt_ux: Enterprise-style Gantt UX for timeline views

[nghorbani]

New module that layers an Enterprise-style Gantt UX on top of web_timeline, opt-in per view via a gantt_ux="true" arch attribute. Everything is patch()-based: web_timeline itself is untouched, and views that do not opt in keep stock look and behavior bit for bit.

We built this for our own project-task planning on Community 19.0 and have been running it in production; it is generic (nothing model-specific), so contributing it here where web_timeline lives.

What it adds

• Row per record under collapsible group bands (record counts, localStorage-persisted collapse). Record names render on the bars and overflow narrow pills via vis's native RangeItem overflow mode; the sidebar carries only the group bands.
• Light theme via CSS design tokens: colors= rules become harmonized pills, weekend + today shading, today's axis label as a pill, day-snapped drags, and a stable window (the stock per-reload fit() is suppressed in gantt mode, so the view never re-zooms after an edit).
• Dependency arrows as Bezier curves predecessor-end to successor-start with hover tooltips:

• Drag-to-link: hover a bar, drag the circular handle onto another bar to create the dependency_arrow link. Client-side validation (self/duplicate/reverse), pending dashed arrow while the write is in flight, success toast with Undo, arrow click / focus+Delete removal with confirmation, and per-record gating when the model exposes an allow_task_dependencies-style field:

• Dependency-aware moves: dragging a bar with dependents asks whether the downstream chain shifts by the same delta, the record moves alone, or the move is cancelled (bar snaps back). One prompt per multi-select gesture.
• Status colors harmonized with the theme:

Design notes

• Group ids are namespaced (grp_<id> bands / rec_<id> rows) while item ids stay raw record ids, so the stock write paths keep working. The one stock coupling (the moveQueue group-by write, which would send project_id: "rec_42" on drags in row-per-record mode) is guarded by a regression test.
• Rows get a deterministic in-parent order: the stock search_read orders by the group-by field only, so database tie order would otherwise shuffle rows on every reload.
• Link handles live in an overlay outside the item DOM, so vis's Hammer instances never see the gesture (no pan/drag/select conflicts); handle placement is selection-aware so vis's resize handles stay fully usable.
• Arrows get transparent 10px hit twins for clicking/focus; a11y: strokes are focusable with aria-labels, Delete removes.

Tests

• 9 hoot suites under static/tests/ (bezier geometry, arch options, group building incl. row-order stability, model remap, controller move/add, drag validation, dependency writes, move cascade).
• Python install/asset-registration tests under tests/.

Related

• [IMP] web_timeline: optional dependency arrows toggle #3621 (dependency-arrows toggle draft) touches the same arrow-drawing area; this module keeps its own arrow layer out of stock code entirely, so the two are independent.

Known limitations (also in ROADMAP)

• m2m group-by uses the first value only (stock item behavior).
• Chart-side dependency creation is pointer-only; the record form remains the keyboard path.
• Dependency-aware moves shift only records in the current search domain.
• No touch support for the hover-born handles yet.