The current layout system uses a complex bidirectional atom architecture
that forces every layout change to round-trip through the backend
WaveObject, even though **the backend never reads this data** - it only
queues actions via `PendingBackendActions`. By switching to a "write
cache" pattern where local atoms are the source of truth and backend
writes are fire-and-forget, we can eliminate ~70% of the complexity
while maintaining full persistence.
----
Every layout change (split, close, focus, magnify) currently follows
this flow:
```
User action
↓
treeReducer() mutates layoutState
↓
layoutState.generation++ ← Only purpose: trigger the write
↓
Bidirectional atom setter (checks generation)
↓
Write to WaveObject {rootnode, focusednodeid, magnifiednodeid}
↓
WaveObject update notification
↓
Bidirectional atom getter runs
↓
ALL dependent atoms recalculate (every isFocused, etc.)
↓
React re-renders with updated state
```
---
## Proposed "Write Cache" Architecture
### Core Concept
```
User action
↓
Update LOCAL atom (immediate, synchronous)
↓
React re-renders (single tick, all atoms see new state)
↓
[async, fire-and-forget] Persist to WaveObject
```
### Key Principles
1. **Local atoms are source of truth** during runtime
2. **WaveObject is persistence layer** only (read on init, write async)
3. **Backend actions still work** via `PendingBackendActions`
4. **No generation tracking needed** (no need to trigger writes)
This was hard to debug. It manifested in nodes that had been collapsed
into their parent. When a node was dragged over the recently-collapsed
node, the placeholder would show the action as if the flex direction was
reversed. It turns out useDrag has trouble tracking changes to the
LayoutNode objects. For now, I am just finding the nodes again in the
computeMoveNode function. Later when I redo the drag system, I will fix
this better.
This PR is a large refactoring of the layout code to move as much of the
layout state logic as possible into a unified model class, with atoms
and derived atoms to notify the display logic of changes. It also fixes
some latent bugs in the node resize code, significantly speeds up
response times for resizing and dragging, and sets us up to fully
replace the React-DnD library in the future.
