Navigate to editor on notebook creation, reduce flicker with double buffering

- Creating a notebook now auto-navigates to its first page editor
  (createNotebook returns ID via callback, triggers onNotebookClick)
- Double-buffered rendering: all layers composited to an off-screen
  bitmap before presenting to the display in a single drawBitmap call.
  Eliminates visible intermediate states (white flash between layers)
  that caused flicker on e-ink displays.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/main/kotlin/net/metacircular/engpad/ui/editor/PadCanvasView.kt

@@ -48,6 +48,11 @@ class PadCanvasView(context: Context) : View(context) {
     private var gridCanvas: Canvas? = null
     private var gridDirty = true
 
+    // --- Compositing bitmap (eliminates flicker by drawing all layers
+    //     off-screen before presenting to the display in one operation) ---
+    private var compositeBitmap: android.graphics.Bitmap? = null
+    private var compositeCanvas: Canvas? = null
+
     // --- In-progress stroke ---
     private var currentPath: Path? = null
     private var currentPaint: Paint? = null
@@ -269,11 +274,24 @@ class PadCanvasView(context: Context) : View(context) {
     }
 
     override fun onDraw(canvas: Canvas) {
-        // Fill entire view with white (no super.onDraw to avoid double-clear flicker)
-        canvas.drawColor(Color.WHITE)
+        val w = width
+        val h = height
+        if (w <= 0 || h <= 0) return
 
-        // Draw page background in canonical space
-        canvas.withMatrix(viewMatrix) {
+        // Ensure compositing bitmap exists at view size
+        if (compositeBitmap == null || compositeBitmap!!.width != w || compositeBitmap!!.height != h) {
+            compositeBitmap?.recycle()
+            compositeBitmap = androidx.core.graphics.createBitmap(w, h)
+            compositeCanvas = Canvas(compositeBitmap!!)
+        }
+
+        val c = compositeCanvas!!
+
+        // Compose all layers off-screen
+        c.drawColor(Color.WHITE)
+
+        // Page background
+        c.withMatrix(viewMatrix) {
             drawRect(
                 0f, 0f,
                 canvasState.pageSize.widthPt.toFloat(),
@@ -282,24 +300,22 @@ class PadCanvasView(context: Context) : View(context) {
             )
         }
 
-        // Draw cached grid
+        // Cached grid
         ensureGrid()
-        gridBitmap?.let { canvas.drawBitmap(it, 0f, 0f, null) }
+        gridBitmap?.let { c.drawBitmap(it, 0f, 0f, null) }
 
-        // Draw completed strokes from backing bitmap (screen resolution)
+        // Completed strokes
         ensureBacking()
-        backingBitmap?.let { canvas.drawBitmap(it, 0f, 0f, null) }
+        backingBitmap?.let { c.drawBitmap(it, 0f, 0f, null) }
 
-        // Draw dynamic elements in canonical space (in-progress stroke, previews, selection)
-        canvas.withMatrix(viewMatrix) {
-            // In-progress stroke
+        // Dynamic elements
+        c.withMatrix(viewMatrix) {
             currentPath?.let { path ->
                 currentPaint?.let { paint ->
                     drawPath(path, paint)
                 }
             }
 
-            // Box preview
             if (isDrawingBox) {
                 boxPreviewPaint.strokeWidth = canvasState.penWidthPt
                 val left = minOf(boxStartX, boxEndX)
@@ -309,7 +325,6 @@ class PadCanvasView(context: Context) : View(context) {
                 drawRect(left, top, right, bottom, boxPreviewPaint)
             }
 
-            // Line preview
             if (isDrawingLine) {
                 val paint = if (canvasState.lineStyle == LineStyle.DASHED) {
                     dashedLinePaint.also { it.strokeWidth = canvasState.penWidthPt }
@@ -323,7 +338,6 @@ class PadCanvasView(context: Context) : View(context) {
                 )
             }
 
-            // Selection highlights
             if (selectedStrokeIds.isNotEmpty()) {
                 for (sr in completedStrokes) {
                     if (sr.id in selectedStrokeIds) {
@@ -334,7 +348,6 @@ class PadCanvasView(context: Context) : View(context) {
                 }
             }
 
-            // Selection rectangle
             if (isSelecting) {
                 val left = minOf(selectionStartX, selectionEndX)
                 val top = minOf(selectionStartY, selectionEndY)
@@ -343,6 +356,9 @@ class PadCanvasView(context: Context) : View(context) {
                 drawRect(left, top, right, bottom, selectionRectPaint)
             }
         }
+
+        // Present the fully composed frame in one operation
+        canvas.drawBitmap(compositeBitmap!!, 0f, 0f, null)
     }
 
     /**

app/src/main/kotlin/net/metacircular/engpad/ui/notebooks/NotebookListScreen.kt

@@ -243,7 +243,9 @@ fun NotebookListScreen(
         CreateNotebookDialog(
             onDismiss = { showCreateDialog = false },
             onCreate = { title, pageSize ->
-                viewModel.createNotebook(title, pageSize)
+                viewModel.createNotebook(title, pageSize) { notebookId ->
+                    onNotebookClick(notebookId)
+                }
                 showCreateDialog = false
             },
         )

app/src/main/kotlin/net/metacircular/engpad/ui/notebooks/NotebookListViewModel.kt

@@ -17,9 +17,10 @@ class NotebookListViewModel(
     val notebooks: StateFlow<List<Notebook>> = repository.getAll()
         .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), emptyList())
 
-    fun createNotebook(title: String, pageSize: String) {
+    fun createNotebook(title: String, pageSize: String, onCreated: (Long) -> Unit = {}) {
         viewModelScope.launch {
-            repository.create(title, pageSize)
+            val id = repository.create(title, pageSize)
+            onCreated(id)
         }
     }