grid-editor

Design

2D layout canvas — pointer interactions (draw/drag/resize/stagingDrag/paint in useInteraction.ts and src/shared/hooks/interactions/), __staging__ stash semantics, layer view modes, half-grid 0.5 snapping, collision/blocked zones, canPlaceBin validation. Load for symptoms like offset drag ghosts, mirrored Y placement, phantom collisions, bins drifting in the stash, laggy drag previews, or half-grid toggle failing.

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Grid Editor: 2D Canvas Interactions

When to use

Changing any pointer interaction: draw, drag, resize, stagingDrag, paint.
Touching placement validation, collision, blocked zones, or displacement-to-staging.
Half-grid (0.5-unit) snapping, sizing, or toggle behavior.
Bugs: offset/mirrored bin positions, phantom collisions, stash rendering glitches, laggy drags.

For CQRS commands/undo behind mutations see the state-and-cqrs skill; for compartment internals see bin-designer; for the 3D preview render systems see three-preview.

Mental model

Orchestrator + mode hooks. src/features/grid-editor/hooks/useInteraction.ts owns pointer events, pointer capture, and RAF throttling, then delegates to per-mode hooks in src/shared/hooks/interactions/ (shared so mobile/tablet shells reuse them). That directory's README is the ground truth for mode behavior — the grid-editor README undersells it.
Y is inverted once, in one place. Grid (0,0) is bottom-left; screen Y=0 is top. useGridCoords.ts (getGridCoords/clampCoords) does the inversion (y = drawer.depth - cellY - 1). Never hand-roll px→grid math.
drag stores a delta; stagingDrag stores an absolute. In a drag Interaction, currentCoord is the constrained delta from startCoord; commit is bin.x + delta. In stagingDrag it is an absolute grid position (null until the pointer reaches the grid). See the Interaction union in src/core/types.ts (~line 301).
Staging is a sentinel, not a layer. STAGING_ID = '__staging__' (src/core/constants.ts) never appears in layout.layers. Every collision/validation loop must skip it: binsCollideResult returns ok(false), getBlockedZones and canPlaceBin skip. Any new placement code that forgets this makes stashed bins phantom-block the grid.
Collision is 3D. Footprint overlap AND vertical overlap, where a bin's zEnd can protrude past its layer into layers above (blocked zones). Layer height is only a default for new bins; canPlaceBin (src/shared/utils/validationPlacement.ts) checks height against drawer.height - layerZStart, never against layer height.
Half-grid changes rendering, not coordinates. Internal coords stay in real grid units at 0.5 steps (snapToHalf). Only Grid.tsx doubles: visualCellSize = (cellSize - gap) / HALF_BIN_SCALE, scale = HALF_BIN_SCALE. Never store scaled coordinates.
CLAUDE.md is stale about layer view modes. layerViewMode ('focus'|'stack'|'all') lives in src/core/store/view.ts, NOT interaction.ts; its only rendering consumer is the 3D preview (src/features/grid-editor/components/Grid/IsometricPreview/useBinsToRender.ts), and the value is also serialized into share links via src/shared/utils/ephemeralState.ts. The 2D canvas's lower-layer ghosting is separate: view.ts showOtherLayers → ghost bins in GridCanvas/GridCanvas.tsx.

Recipes

Add or change a placement validation reason

Extend ValidationReason in src/core/types.ts; return it from canPlaceBin in src/shared/utils/validationPlacement.ts (check order: bounds → layer → height → blocked_zone → collision).
Map it to a user message in getPlacementErrorMessage in src/shared/utils/validation.ts; add keys per the i18n-changes skill.
Keep the layerId === STAGING_ID skip and excludeBinIds (group drags) working.
Update switches over the union — src/features/grid-editor/components/Grid/Overlay/Overlay.tsx — or pnpm run check:exhaustiveness blocks the commit.

Change staging/displacement semantics

Store side: src/core/store/layout/drawerActions.ts (drawer shrink) and layerActions.ts (layer delete) both set layerId = STAGING_ID and preserve x/y.
Mirror the change in the CQRS replay handlers src/core/cqrs/v2/domain/drawer/updateDrawer.ts and src/core/cqrs/v2/domain/layer/deleteLayer.ts — divergence silently corrupts undo/collab replay (see state-and-cqrs skill).
Stash visuals: src/features/staging/components/Staging/Staging.tsx + src/features/staging/utils/packing.ts. Packed positions are recomputed per render — never persist them back to bins.

Modify half-grid behavior

Snapping helpers: snapToHalf/snapToGrid/isFractional in src/core/constants.ts. Pointer conversion + clamping: useGridCoords.ts. Min draw size (0.5 : 1): src/shared/hooks/interactions/useDrawInteraction.ts. Smart-snap step: useDragInteraction.ts / useStagingDragInteraction.ts (step = halfGridMode ? 0.5 : 1).
Rendering is separate: Grid.tsx visual math and src/features/grid-editor/utils/fractionalPixels.ts.
If validity of existing layouts changes, update validateHalfGridModeToggle in src/shared/utils/halfGridConstraints.ts (the toggle-off guard in src/core/store/halfGridMode.ts).

Add a new interaction mode or drop target

Study an existing mode hook in src/shared/hooks/interactions/ and its README; wire it into useInteraction.ts following the ref-of-handlers pattern (drawModeRef etc. synced in useLayoutEffect).
Extend the Interaction union in src/core/types.ts; run pnpm run check:exhaustiveness.
Drop targets are geometric, not DOM drop events: Staging.tsx listens to document pointermove, compares client coords to its bounding rect, sets interactionStore.dropTarget = 'staging'; useDragInteraction.handleUp reads dropTarget before grid placement. A new target needs both sides.

Verification

pnpm run test:run src/features/grid-editor
pnpm run test:run src/shared/hooks/interactions
pnpm run test:run src/core/store/layout.scenario.displacement
pnpm run test:run src/core/store/layout.scenario.half-grid
pnpm run check:exhaustiveness
pnpm run test:e2e

Interaction behavior specs live in src/features/grid-editor/hooks/useInteraction.{core,drag,draw,resize,staging}.test.ts; displacement semantics in src/core/store/layout.scenario.displacement.test.ts. Always test with a half-grid layout (x.5 coords) in addition to integer grids.

Traps

Symptom	Cause	Fix
Bins mirrored vertically / drag ghost offset from cursor	Y inversion missed or applied twice	Route all conversion through `useGridCoords` `getGridCoords`/`clampCoords`
Group drop lands near (0,0)	Treated drag `currentCoord` as absolute	It is the delta; commit as `bin.x + currentCoord.x` (see `useDragInteraction.handleUp`)
"Collision"/"blocked zone" on an empty-looking layer	Lower-layer bin protrudes upward, or new code forgot the `STAGING_ID` skip	Inspect `getBlockedZones` output; skip `bin.layerId === STAGING_ID` in every bin loop
Drag preview lags / validation storms	Heavy work in the unthrottled draw/paint path	Keep expensive validation in the RAF-throttled branch of `useInteraction` (`processHeavyMove`); draw/paint stays cheap
Handlers act on stale state	Mode handlers captured directly in document listeners	Use the existing ref-sync pattern; read live state via `useInteractionStore.getState()` inside handlers
Commit logic never ran after a drag	`pointercancel` (OS gesture steal) discards without calling `handleUp`	Never assume `handleUp` runs; keep all commit logic in `handleUp`, nothing partial before it
Interaction stuck or cleared twice	Mode `handleUp` called `setInteraction(null)` on normal completion	Parent `useInteraction` clears it; only early-return branches (swap, staging drop) clear themselves
Staged bins drift above stash Y=0	CSS `repeat(N, …)` drops non-integer N	Keep `Staging.tsx` `gridHeight = Math.ceil(...)` and `StagingBin.tsx` `alignSelf: 'end'` for fractional depth — `git show 5517327ae`
Half-grid toggle-off silently fails	`toggleHalfGridMode` returns Err while fractional bins exist	Check the Result; resize/delete fractional bins first. localStorage key `gridfinity-half-bin-mode` is legacy — never rename without a migration
Selection empties on layer switch	`setActiveLayer` clears `selectedBinIds` by design (`src/core/store/selection.ts`)	Not a bug; cross-layer selection is a product decision
Blocked zones stale mid-drag	`getBlockedZones` caches by reference equality of bins/layers arrays (`src/shared/utils/collision.ts`)	Never mutate layout arrays in place; Zustand+Immer must produce new references
Half-grid ON changes bin-designer defaults	Intentional: seeds `base.halfSockets = true` on new designs	See `halfGridMode.ts` docstring; bin-designer skill covers the designer side

Other verified behaviors to respect: multi-select drag is all-or-nothing unless snapGroupDelta rescues it (Ctrl disables snap; ctrlKeyRef resets on window blur and visibilitychange because keyup never fires); getGridCoords uses cellX/cellY = -1 as the fractional-edge sentinel; mlTracking.trackMove/trackPlacement must fire before mutations (they need pre-mutation positions); moved bins keep their height across layers/staging by design; exploded 3D view forces layerViewMode='all' (view.ts coupling).