The layout snapshot is the only layout state, groups and columns included

jolly-dock-layout keeps pane placement, dock size and collapse, floating geometry, pane folds and folder state in one LayoutSnapshot. A drag or keyboard move computes the next snapshot with a pure function in layout.ts, then LayoutProjection writes it onto the DOM. Nothing reads the DOM back to learn what changed. Docks, windows and panes report their own edits as a typed LayoutChange.

Keys and reconciliation are unchanged, so ADR-0022 still holds.

A dock state is an ordered list of groups, { panes, active }, and a lone pane is a group of one. jolly-pane-group is a projection of a group with two or more panes: LayoutProjection creates it, reuses it, and replaces it with its last pane. Tabs that move between docks have to be panes the layout can see, so an editor that wants movable tabs authors one light-DOM jolly-pane per tab rather than a jolly-tabs.

A double dock keeps its second column in the same DockState, as secondary next to groups, and the projection moves a slot there with slot="secondary". Size, collapse and visibility stay on the dock, so both columns resize, collapse and hide together. Transitions address a column with { dock, column }; a plain dock key means the primary column. A dock whose primary column empties takes the secondary groups, so no empty strip stays against the screen edge.

Considered Options

• Reading the DOM after each mutation. The previous design. Every layout property needed a write mapping, a read mapping and a side cache, and a slot change could reconcile a stale snapshot over a move that had just been made.
• Children writing straight to storage. Two owners for one persisted document.
• A flat pane list plus a grouping annotation. Every transition would have to keep group members contiguous, and parsing would have to repair lists that are not.
• Groups as authored containers only. Dropping a pane on another pane could not create a group without the application reacting to the drop.
• Letting jolly-tabs hold panes. Its jolly-tab panels have their own selection state, a second owner of the active tab.
• Two authored docks side by side for a double dock. Each has its own handle, size and collapse state, and nothing keeps the widths equal.

Consequences

A property set on a dock or window from outside, without an interaction that reports it, is not in the snapshot until the next sync(). Clamping a window to the viewport is a display concern and is not recorded.

Visibility is derived from the snapshot, so jolly-pane-visibility needs no DOM measurement and ignores application CSS that hides a dock.

A floating window holds one pane; dragging a tab out floats that pane. Consumers comparing PanePlacement values must compare column too. The second column has no splitter, and the columns are always equal.