fix: PDF finger ink, chrome UX, OneNote pens, and pen physics

Wire finger drawing on PDF without breaking pinch; auto-hide page scrubber and fix bounce; share sticky tools with resize and per-page remember; side-button select; separate pen slots with colors; rnote pressure shapes plus tip-velocity width and lower stroke latency.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/editor/engine/brush.dart

@@ -135,16 +135,15 @@ class BrushProfile {
 /// highlighter ≈ 0.02 so the existing pen/highlighter visuals are PRESERVED as
 /// the fountainPen/highlighter presets (no regression).
 const Map<BrushKind, BrushProfile> kBrushPresets = {
-  // Fountain pen — spec §4: size~6, thinning 0.9, smoothing 0.55,
-  // streamline 0.45, simulatePressure false, taper on, pressure pre-warped to
-  // p² (Pow2 / quadratic = pressureGamma 2.0). Solid ink (opacity 1.0).
+  // Fountain pen — Surface feel: lower streamline (less lag), higher thinning
+  // for expressive width, pressure pre-warped to p² (Pow2 / quadratic).
+  // Solid ink (opacity 1.0). See also tipVelocityWidthScale (ink starvation).
   BrushKind.fountainPen: BrushProfile(
     kind: BrushKind.fountainPen,
     baseWidthFraction: 0.006,
     pressureGamma: 2.0,
-    // Was 0.9 — too aggressive on short CJK strokes (width collapses mid-glyph).
-    pfThinning: 0.65,
-    pfStreamline: 0.4,
+    pfThinning: 0.75,
+    pfStreamline: 0.22,
     pfSmoothing: 0.5,
     simulatePressure: false,
     capStart: true,
@@ -154,15 +153,14 @@ const Map<BrushKind, BrushProfile> kBrushPresets = {
     opacity: 1.0,
     blendMultiply: false,
   ),
-  // Ballpoint — Krita-inspired "ink pen": near-constant width, SOLID opacity.
-  // Pressure modulates WIDTH slightly (thinning 0.15), NOT alpha — translucent
-  // srcOver stacking looked like accidental multiply when strokes overlapped.
+  // Ballpoint — near-constant width, SOLID opacity. Lower streamline (~0.35)
+  // for lower latency; thinning 0.12 keeps width almost flat.
   BrushKind.ballpoint: BrushProfile(
     kind: BrushKind.ballpoint,
     baseWidthFraction: 0.0022,
     pressureGamma: 1.0,
-    pfThinning: 0.15,
-    pfStreamline: 0.55,
+    pfThinning: 0.12,
+    pfStreamline: 0.35,
     pfSmoothing: 0.5,
     simulatePressure: false,
     capStart: true,
@@ -171,18 +169,14 @@ const Map<BrushKind, BrushProfile> kBrushPresets = {
     opacity: 1.0,
     blendMultiply: false,
   ),
-  // Highlighter — spec §4: size~22, thinning 0.0 (constant width),
-  // smoothing 0.4, streamline 0.5, square (uncapped) ends, translucent +
-  // multiply build-up. opacity 0.35 / blendMultiply true are APPLIED via
-  // resolveStrokePaint: the 0.35 is multiplied INTO the color's existing alpha
-  // (the capture path ships a 0x80 / 50% translucent color), and the stroke
-  // composites with BlendMode.multiply (cross-stroke overlap darkens = marker).
+  // Highlighter — flat width (thinning 0), square (uncapped) ends, translucent +
+  // multiply build-up. streamline 0.3 for a bit less lag on broad strokes.
   BrushKind.highlighter: BrushProfile(
     kind: BrushKind.highlighter,
     baseWidthFraction: 0.02,
     pressureGamma: 1.0,
     pfThinning: 0.0,
-    pfStreamline: 0.5,
+    pfStreamline: 0.3,
     pfSmoothing: 0.4,
     simulatePressure: false,
     capStart: false,
@@ -191,15 +185,13 @@ const Map<BrushKind, BrushProfile> kBrushPresets = {
     opacity: 0.35,
     blendMultiply: true,
   ),
-  // Pencil — Krita-inspired: soft graphite, moderate translucency via √p, but
-  // NEVER multiply blend (only highlighter uses multiply). Cap ~0.88 so overlaps
-  // darken gently under srcOver without turning into marker blobs.
+  // Pencil — soft graphite via √p, moderate translucency. streamline 0.25.
   BrushKind.pencil: BrushProfile(
     kind: BrushKind.pencil,
     baseWidthFraction: 0.003,
     pressureGamma: 0.5,
     pfThinning: 0.45,
-    pfStreamline: 0.35,
+    pfStreamline: 0.25,
     pfSmoothing: 0.45,
     simulatePressure: false,
     capStart: true,