feature: Eliah layout tree — window as a nested-TPaned splitter tree

• Type: feature (Track B)
• Status: done
• Track: B
• Parent: feature-eliah-shell
• Opened: 2026-06-23

Goal

Replace Eliah's absolute gtk_fixed + manual Relayout pane positioning with a tree of TPaned whose leaves are pane containers. This is the foundation the rest of the shell stands on.

Why

Today apps/ide/eliah/main.pas sizes every pane with absolute SetBounds inside the form's gtk_fixed, reflowing by hand on resize. That cannot express draggable splitters, collapse, or saved perspectives. A TPaned tree gives real drag for free and makes layout data (a tree + ratios) instead of code.

Scope

• Define a pane-container abstraction (TIdePane or similar): a leaf holding exactly one view today (tree / editor / output / errors / designer / inspector / palette). It owns a header area (for the future collapse chevron + title) and a content slot. Keep it a slot so a tab strip can replace the single view later — do not hardcode one-view-per-leaf.
• Build the current Eliah layout as a TPaned tree, e.g.: HPaned[ VPaned[ tree , errors ] | HPaned[ center , VPaned[ output , inspector ] ] ] (exact shape TBD — match the present panes).
• Drop the absolute-coordinate Relayout; let GtkPaned own sizing. Keep the form-level resize only where genuinely needed.
• Children parent into panes via the existing TGtk3WidgetSet.SetParent TPaned branch (pack1/pack2) — already landed with TPaned.

Non-goals (later milestones)

• Collapse/restore strips (feature-eliah-pane-collapse).
• Saved perspectives / presets / priority-compacting (feature-eliah-perspectives).

Acceptance

Eliah renders the same panes as today but every divider is a draggable splitter. Resizing the window behaves sanely (GtkPaned reflow). --smoke green, gui_suite green, apps/ide/test.sh (garin) green, screenshot confirms all panes visible with draggable handles. No absolute gtk_fixed pane math left in the main layout path.

Risks / notes

• This is the big rewrite flagged in the layout discussion. Do it as one focused change; verify with a screenshot (headless --smoke does NOT prove render).
• A multi-child column (e.g. tree above errors) is a nested TPaned, since GtkPaned holds exactly two children. Expect a tree a few levels deep.
• Menubar/toolbar stay outside the paned tree (top of the form vbox), as now.
• May surface TPaned gaps (min-size, initial ratios before realize) → file PCL tickets, keep app code idiomatic.

Log

• 2026-06-23 — filed (milestone 1 of feature-eliah-shell).

Log

• 2026-06-24 — DONE. Eliah's window is now one nested-TPaned tree (RootPaned): HPaned[ colLeft(V: tree/errors) | HPaned[ colCenter(V: editor/output) | colRight(V: designer / colInspector(V: props/valueEdit)) ] ]. Children parent into leaves via the existing SetParent TPaned branch. Absolute per-pane Relayout math removed — Relayout now only resizes RootPaned; GtkPaned owns the splits. Handle positions seeded once on the first allocation (OnFormResize), since GtkPaned clamps a position set before it has a size. Smoke asserts root reflow; gui_suite + garin(92) green; screenshot (Xvfb :99) confirms all panes render with draggable handles. Also added devdocs/developer/gui-testing.md (Xvfb-based GUI testing, no foreground grab).