Re-split library and preview when the editor is hidden; remember the editor's width

docs/architecture/10-interaction-and-feedback.md

@@ -181,6 +181,23 @@ lives in [04 · Routing & Global Events](04-routing-and-events.md).
   now has a home — the hidden-pane state landed in M6 with the toggle strip (below). The strip
   owns show/hide; wiring the splitter's Enter to it is an optional convenience, not required.
 
+**Editor-hidden layout.** Pane model: the two **side** panes carry stored widths; the **editor
+is the flex filler** (no stored width while shown). Show/hide is layout-aware only for the
+editor — hiding **captures** the width it had (`capturedEditorWidth`), showing **pins it back**
+and re-splits the rest at the side panes' ratio (`shownSideWidths`); side panes just flip
+visibility and flex-fill the freed space (spec §01A). So `togglePane` needs `panesInner`
+(panes-row width minus the toggle strip, measured by `PaneToggleStrip`) for the editor only.
+With the editor hidden, library and preview become adjacent and share a **second splitter**,
+`PaneSplitHandle` — separate from `ResizeHandle` because the geometry differs: `ResizeHandle`
+resizes one side pane while the editor absorbs the change (the opposite side is untouched);
+`PaneSplitHandle` re-proportions **two** panes with no filler between them (editor-hidden, the
+side panes render `flex: width 1 0` and share the span), the pure `splitLibraryWidth()` holding
+each above its min. **valuenow trade-off:** it reports the library's 0–100 position
+(`splitValue()`) from the **stored ratio**, deliberately container-independent — so unlike
+`ResizeHandle` it does **not** observe the container, and `aria-valuenow` can drift from the
+rendered position after a window enlargement (worst near the extremes). Drag/keyboard read live
+`clientWidth`, so resizing itself stays accurate; only the announced value drifts.
+
 _(Consulted via `/council` → WAI-ARIA APG `windowsplitter`. This bullet is the contract;
 cite it, not the APG file.)_
 

docs/spec/01-application-shell.md

@@ -13,10 +13,10 @@ Astrolabe is a single-screen workspace. Below a fixed top header sits a three-pa
 Behavior:
 
 - All three panes are visible by default, laid out side by side in the order library, editor, preview.
-- Adjacent panes are separated by a vertical drag handle. The user can drag a handle left/right to resize the two panes it sits between; the rest of the layout is unaffected.
+- Adjacent panes are separated by a vertical drag handle. The user can drag a handle left/right to resize the two panes it sits between; the rest of the layout is unaffected. When the editor is hidden, the library and preview become adjacent and a single handle between them re-splits the freed space.
 - Each pane enforces a minimum width while resizing, so a pane cannot be dragged to nothing.
 - Each pane can be individually shown or hidden via a persistent **toggle strip** (a narrow vertical strip of toggle buttons, one per pane, each indicating whether its pane is currently shown). The toggle strip also contains a shortcut button that opens the Datasets manager.
-- When a pane is hidden, the remaining visible panes expand to fill the freed space, redistributing proportionally to their remembered widths. When a previously hidden pane is shown again, it returns at its remembered width.
+- When a pane is hidden, the remaining visible panes expand to fill the freed space, redistributing proportionally to their remembered widths. When a previously hidden pane is shown again, it returns at its remembered width. The center editor is normally the flex filler with no fixed width, so it remembers the width it had at the moment it was hidden and reclaims that width when shown again; the library and preview then keep the split ratio they were left at while it was hidden.
 - Hiding all panes is permitted; the toggle strip remains available to bring panes back.
 - Pane widths and per-pane visibility persist locally across sessions and are restored on next load. The app remembers a pane's preferred width even while it is hidden, so re-showing it restores that width rather than an arbitrary one.