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+# Splitscreen Tools Architecture
+
+Issue: #107
+
+This document explains the splitscreen tools system in TPEN transcription interfaces: tool types, lifecycle, event communication, and integration patterns.
+
+## Purpose
+
+Provide a pluggable system for transcription-adjacent tools that appear alongside the main workspace without disrupting the core workflow.
+
+## Scope
+
+In scope:
+
+1. Tool types and their visual/behavioral characteristics
+2. Tool activation and dismissal lifecycle
+3. Event communication between tools and interface
+4. Tool configuration via TPEN.activeProject.tools
+5. Integration points with the transcription interface shell
+
+Out of scope:
+
+1. Individual tool implementation details (covered in component-specific docs)
+2. Tool authentication/authorization beyond project-level permissions
+3. Non-transcription interface tool usage patterns
+
+## Tool Types
+
+The system supports four distinct tool presentation modes:
+
+### 1. Split Pane Tools
+
+**Visual behavior**: Appear to the right of the main interface, creating a resizable horizontal division.
+
+**Use cases**: Full-featured tools that need substantial space (dictionaries, references, image viewers).
+
+**Current implementation**: 
+- Managed by tpen-simple-transcription interface state
+- Activated via splitscreen-toggle event
+- Location identifier: `"pane"`
+
+**References**:
+- [components/simple-transcription/index.js](../simple-transcription/index.js) (interface orchestration)
+- [components/splitscreen-tool/index.js](../splitscreen-tool/index.js) (tool selector dropdown)
+
+### 2. Sidebar Tools
+
+**Visual behavior**: Fixed-width panel that slides in from the right, non-resizable.
+
+**Use cases**: Compact tools, configuration panels, quick reference materials.
+
+**Current implementation**:
+- Location identifier: `"sidebar"`
+- Shares activation mechanism with pane tools
+
+### 3. Drawer Tools
+
+**Visual behavior**: Floating tray that slides over content, dismissible without losing context.
+
+**Use cases**: Magnifier, inspector, temporary overlays.
+
+**Current implementation**:
+- [components/magnifier-tool/index.js](../magnifier-tool/index.js)
+- Activated via specific tool buttons or shortcuts
+- Does not use split-screen layout
+
+### 4. Dialog Tools
+
+**Visual behavior**: Modal overlay that interrupts workflow until dismissed.
+
+**Use cases**: Critical actions requiring full attention, configuration wizards.
+
+**Current implementation**:
+- Uses GUI confirm/alert system
+- Not directly tied to project.tools array
+
+## Tool Configuration
+
+Tools are defined in `TPEN.activeProject.tools` array with this structure:
+
+```javascript
+{
+  toolName: "dictionary",        // Unique identifier
+  label: "Dictionary",            // Display name
+  location: "pane",               // Type: "pane" | "sidebar" | "drawer" | "dialog"
+  url: "https://example.com/tool.js", // Optional: URL for module script or iframe src
+  custom: {
+    enabled: true,                // Optional: defaults to true if omitted
+    tagName: "tpen-dictionary",   // Optional: custom element tag name (requires url)
+    // Tool-specific configuration
+  }
+}
+```
+
+The rendering path is determined by these two properties:
+
+1. If `custom.tagName` AND `url` → load `url` as a module script, then render `<tagName>`
+2. If `url` AND no `tagName` AND `location === 'pane'` → render as `<iframe src="url">`
+3. Otherwise → fallback message
+
+**Filtering rules**:
+- Tools with `custom.enabled === false` are excluded
+- Only tools with `location === "pane"` or `location === "sidebar"` appear in splitscreen selector
+- Drawer and dialog tools use separate activation mechanisms
+
+## Component Inventory
+
+Primary components:
+
+1. **tpen-workspace-tools** - Toolbar that provides tool activation buttons
+   - Renders quicktype, magnifier, page tool, splitscreen selector
+   - Requires TOOLS ANY view access
+   - Reference: [components/workspace-tools/index.js](index.js)
+
+2. **tpen-splitscreen-tool** - Dropdown selector for pane/sidebar tools
+   - Filters project.tools by location and enabled state
+   - Dispatches splitscreen-toggle event on selection
+   - Reference: [components/splitscreen-tool/index.js](../splitscreen-tool/index.js)
+
+3. **tpen-magnifier-tool** - Drawer-style image magnification overlay
+   - Activated via toolbar button
+   - Dismissible via Escape key or close button
+   - Reference: [components/magnifier-tool/index.js](../magnifier-tool/index.js)
+
+4. **Individual tool implementations** (e.g., tpen-page-tool, tpen-quicktype-tool)
+   - Each tool is responsible for its own rendering and behavior
+   - Reference: [components/page-tool/index.js](../page-tool/index.js)
+
+## Tool Lifecycle
+
+### Activation
+
+1. User selects tool from splitscreen dropdown or clicks toolbar button
+2. `tpen-splitscreen-tool` dispatches `splitscreen-toggle` DOM event (bubbles) with `{ selectedTool: toolName }`
+3. `tpen-splitscreen-tool` dispatches `tpen-${previousTool}-hide` (if switching) and `tpen-${toolName}-show` via `TPEN.eventDispatcher` — these happen synchronously in the same handler as step 2
+4. Interface shell (`tpen-simple-transcription`) receives the bubbled `splitscreen-toggle` event
+5. Interface updates state, toggles split-screen layout, and loads tool content
+
+### Dismissal
+
+1. User clicks close button, presses Escape key, or an external component dispatches `tools-dismiss` via `TPEN.eventDispatcher`
+2. Interface shell calls `closeSplitscreen()` directly — sets `isSplitscreenActive = false` and collapses layout
+3. **Note**: No `tpen-{toolName}-hide` event is dispatched on panel close (only on tool switch). Tools that need cleanup on dismiss should listen for the `tools-dismiss` eventDispatcher event.
+
+### Tool Switching
+
+1. When changing tools, previous tool receives hide event
+2. New tool receives show event
+3. Split-screen layout remains active
+
+## Event Contracts
+
+Primary events:
+
+| Event | Direction | Detail | Purpose |
+|-------|-----------|--------|---------|
+| `splitscreen-toggle` | Selector → Interface (DOM, bubbles) | `{ selectedTool: string }` | Activate/switch split-screen tool |
+| `tools-dismiss` | External → Interface (eventDispatcher) | none | Request interface to close split-screen panel |
+| `tpen-{toolName}-show` | Selector → Tool (eventDispatcher) | none | Notify tool it’s visible (on tool switch) |
+| `tpen-{toolName}-hide` | Selector → Tool (eventDispatcher) | none | Notify previous tool it’s hidden (on tool switch only) |
+
+## Data Ownership
+
+| Data | Owner | Readers | Mutation |
+|------|-------|---------|----------|
+| Available tools list | TPEN.activeProject.tools | workspace-tools, splitscreen-tool | Project load/update |
+| Active tool selection | tpen-simple-transcription state | Interface shell | splitscreen-toggle event |
+| Split-screen open/closed | tpen-simple-transcription state | Interface shell, CSS classes | UI interactions |
+| Tool-specific state | Individual tool components | Tool component only | Tool-internal logic |
+
+## Minimal Knowledge Boundaries
+
+### Interface Shell
+
+- Knows: split-screen layout state, active tool name
+- Does not know: tool-specific rendering logic or internal state
+
+### Workspace Tools
+
+- Knows: available tools from project configuration
+- Does not know: split-screen layout mechanics
+
+### Splitscreen Tool Selector
+
+- Knows: how to filter and display tool options
+- Does not know: tool implementation details or split-screen layout
+
+### Individual Tools
+
+- Know: their own rendering and behavior
+- Do not know: interface layout mechanics or other tools' state
+
+## Integration with Legacy Tools
+
+### iframe Tools (TPEN 2.8 Pattern)
+
+Legacy iframe tools are split-pane tools using bidirectional `postMessage` communication:
+
+- Interface → Tool: sends `MANIFEST_CANVAS_ANNOTATIONPAGE_ANNOTATION`, `CANVASES`, `CURRENT_LINE_INDEX`, and `SELECT_ANNOTATION` messages on load and line changes
+- Tool → Interface: sends `CURRENT_LINE_INDEX`, `RETURN_LINE_ID`, `SELECT_ANNOTATION`, and `NAVIGATE_TO_LINE` messages to request line navigation
+- Origin validation is enforced for incoming messages
+
+See the workspace components architecture for the full postMessage protocol.
+
+**Migration path**: Consider replacing with native components that can use TPEN.eventDispatcher for direct communication.
+
+### Page Options Pattern
+
+Legacy sidebar-like collection of multiple sub-tools:
+
+- Current pattern: separate tools for each option
+- Alternative pattern: composite tool component with internal sub-tool management
+
+## Performance Considerations
+
+1. **Lazy loading**: Tools should only render when activated
+2. **Cleanup**: Tools must clean up event listeners and resources on dismissal
+3. **Caching**: Tools may cache data but must handle project/page changes
+4. **Resizing**: Split-pane resizing should use requestAnimationFrame for smooth dragging
+
+## Accessibility
+
+1. Keyboard shortcuts for common tools (magnifier, quicktype)
+2. Escape key dismisses active tools
+3. Focus management when tools open/close
+4. ARIA labels on tool activation buttons
+
+## Future Enhancements
+
+Potential improvements tracked separately:
+
+1. Standardized tool plugin API for third-party tools
+2. Tool state persistence across sessions
+3. Multi-tool layouts (multiple tools visible simultaneously)
+4. Tool keyboard shortcut customization
+5. Tool marketplace or registry system
+
+## Integration Points
+
+**Note**: The splitscreen patterns documented here are implemented in both `tpen-simple-transcription` (`components/simple-transcription/index.js`) and the standalone transcription interface (`interfaces/transcription/index.js`). Both follow the same event contracts and lifecycle.
+
+## Related Documentation
+
+- [Transcription Interface Architecture](../simple-transcription/ARCHITECTURE.md)
+- [Transcription Interface Diagrams](../simple-transcription/DIAGRAMS.md)
+- [Design Epic](../simple-transcription/DESIGN_EPIC.md)
+
+## Change Log
+
+- 2026-03-09: Initial splitscreen tools architecture document created from issue #107 scope.