@@ -562,6 +562,22 @@ enum SliderTrackShapeEnum {
   }
 }
 
+/// Represents the shape of a slider's thumb.
+enum SliderThumbShapeEnum {
+  /// A circle of the slider's thumb radius.
+  circle,
+
+  /// A rectangle of the slider's thumb width and height with rounded
+  /// corners, like a fader cap.
+  roundedRect;
+
+  /// Display-able string representation of the enum.
+  String get prettify => switch (this) {
+    SliderThumbShapeEnum.circle => 'Circle',
+    SliderThumbShapeEnum.roundedRect => 'Rectangle',
+  };
+}
+
 /// When a slider shows its value indicator. Mirrors Flutter's
 /// `ShowValueIndicator`.
 enum ShowValueIndicatorC {
 
@@ -124,11 +124,23 @@ class MotionModel with Equatable, SerializableMixin {
   /// pulse, a blinking cursor.
   final MotionLoop loop;
 
-  /// Animates from rest to these values when the node first appears, over
-  /// [durationMs], instead of starting there: a record sliding out of its
-  /// sleeve when the screen opens.
+  /// Animates from rest (or from [enterFrom]) to these values when the node
+  /// first appears, over [durationMs], instead of starting there: a record
+  /// sliding out of its sleeve when the screen opens.
   final bool enter;
 
+  /// Where the entrance starts instead of rest: its offset, scale, opacity,
+  /// rotation and tilts (its other settings are ignored). Setting it plays
+  /// an entrance even when [enter] is false: a record fading and settling in
+  /// from `{"scale": 1.15, "opacity": 0}`.
+  final MotionModel? enterFrom;
+
+  /// Plays the entrance again, from [enterFrom] or rest, whenever this
+  /// changes. Meant to be bound through `motion.replayKey` in
+  /// [BaseNode.variables] to any value, for example `${playingId}`, so
+  /// loading another record replays its entrance.
+  final String? replayKey;
+
   /// Waits this long, in milliseconds, before a transition, entrance or
   /// loop starts, so motions can be staggered.
   final int delayMs;
@@ -151,6 +163,8 @@ class MotionModel with Equatable, SerializableMixin {
     this.spinRampMs = 800,
     this.loop = MotionLoop.none,
     this.enter = false,
+    this.enterFrom,
+    this.replayKey,
     this.delayMs = 0,
   });
 
@@ -170,6 +184,10 @@ class MotionModel with Equatable, SerializableMixin {
     'spin',
   ];
 
+  /// The name of the binding that replays the entrance when its value
+  /// changes; see [replayKey].
+  static const String replayKeyProperty = 'replayKey';
+
   /// The key in [BaseNode.variables] that binds [property].
   static String variableKey(String property) => 'motion.$property';
 
@@ -183,6 +201,24 @@ class MotionModel with Equatable, SerializableMixin {
       rotateX != 0 ||
       rotateY != 0;
 
+  /// Whether the node animates in when it first appears.
+  bool get hasEntrance => enter || enterFrom != null;
+
+  /// Where the entrance starts: these settings with the transform of
+  /// [enterFrom], or at rest when there is none.
+  MotionModel get entranceStart => switch (enterFrom) {
+    null => atRest,
+    MotionModel from => copyWith(
+      translateX: from.translateX,
+      translateY: from.translateY,
+      scale: from.scale,
+      opacity: from.opacity,
+      rotate: from.rotate,
+      rotateX: from.rotateX,
+      rotateY: from.rotateY,
+    ),
+  };
+
   /// The value of [property] in this model.
   double valueOf(String property) => switch (property) {
     'translateX' => translateX,
@@ -214,6 +250,10 @@ class MotionModel with Equatable, SerializableMixin {
     int? spinRampMs,
     MotionLoop? loop,
     bool? enter,
+    MotionModel? enterFrom,
+    bool clearEnterFrom = false,
+    String? replayKey,
+    bool clearReplayKey = false,
     int? delayMs,
   }) => MotionModel(
     translateX: translateX ?? this.translateX,
@@ -232,6 +272,8 @@ class MotionModel with Equatable, SerializableMixin {
     spinRampMs: spinRampMs ?? this.spinRampMs,
     loop: loop ?? this.loop,
     enter: enter ?? this.enter,
+    enterFrom: clearEnterFrom ? null : enterFrom ?? this.enterFrom,
+    replayKey: clearReplayKey ? null : replayKey ?? this.replayKey,
     delayMs: delayMs ?? this.delayMs,
   );
 
@@ -277,6 +319,8 @@ class MotionModel with Equatable, SerializableMixin {
     spinRampMs,
     loop,
     enter,
+    enterFrom,
+    replayKey,
     delayMs,
   ];
 
 
@@ -22,6 +22,12 @@ const double kSliderDefaultTickMarkRadius = 1;
 /// Default radius of slider's thumb.
 const double kSliderDefaultThumbRadius = 10;
 
+/// Default width and height of a slider's rectangular thumb.
+const double kSliderDefaultThumbSize = 20;
+
+/// Default corner radius of a slider's rectangular thumb.
+const double kSliderDefaultThumbCornerRadius = 4;
+
 /// Default radius of slider's overlay.
 const double kSliderDefaultOverlayRadius = 24;
 
@@ -191,9 +197,26 @@ class SliderProperties extends CustomProperties with FieldsHolder {
   /// Whether to allow non-whole numbers as min or max values of slider.
   bool allowFractionalPoints;
 
-  /// Radius of slider's thumb.
+  /// Radius of slider's thumb when [thumbShape] is
+  /// [SliderThumbShapeEnum.circle].
   double thumbRadius;
 
+  /// Shape of slider's thumb: a circle of [thumbRadius], or a rounded
+  /// rectangle of [thumbWidth] by [thumbHeight] with [thumbCornerRadius]
+  /// corners.
+  SliderThumbShapeEnum thumbShape;
+
+  /// Width of a rectangular thumb as drawn on screen, across a vertical
+  /// slider's track and along a horizontal one's.
+  double thumbWidth;
+
+  /// Height of a rectangular thumb as drawn on screen, along a vertical
+  /// slider's track and across a horizontal one's.
+  double thumbHeight;
+
+  /// Corner radius of a rectangular thumb.
+  double thumbCornerRadius;
+
   /// Whether to show slider's thumb.
   bool showThumb;
 
@@ -266,6 +289,10 @@ class SliderProperties extends CustomProperties with FieldsHolder {
     this.valueIndicatorTextColor = ColorRGBA.white,
     this.valueIndicatorFontSize = 14,
     this.thumbRadius = kSliderDefaultThumbRadius,
+    this.thumbShape = SliderThumbShapeEnum.circle,
+    this.thumbWidth = kSliderDefaultThumbSize,
+    this.thumbHeight = kSliderDefaultThumbSize,
+    this.thumbCornerRadius = kSliderDefaultThumbCornerRadius,
     this.showThumb = true,
     this.trackShape = SliderTrackShapeEnum.roundedRectangle,
     this.tickMarkRadius = kSliderDefaultTickMarkRadius,
@@ -304,6 +331,10 @@ class SliderProperties extends CustomProperties with FieldsHolder {
     ColorRGBA? valueIndicatorTextColor,
     double? valueIndicatorFontSize,
     double? thumbRadius,
+    SliderThumbShapeEnum? thumbShape,
+    double? thumbWidth,
+    double? thumbHeight,
+    double? thumbCornerRadius,
     bool? showThumb,
     SliderTrackShapeEnum? trackShape,
     double? tickMarkRadius,
@@ -343,6 +374,10 @@ class SliderProperties extends CustomProperties with FieldsHolder {
       valueIndicatorFontSize:
           valueIndicatorFontSize ?? this.valueIndicatorFontSize,
       thumbRadius: thumbRadius ?? this.thumbRadius,
+      thumbShape: thumbShape ?? this.thumbShape,
+      thumbWidth: thumbWidth ?? this.thumbWidth,
+      thumbHeight: thumbHeight ?? this.thumbHeight,
+      thumbCornerRadius: thumbCornerRadius ?? this.thumbCornerRadius,
       showThumb: showThumb ?? this.showThumb,
       trackShape: trackShape ?? this.trackShape,
       tickMarkRadius: tickMarkRadius ?? this.tickMarkRadius,
@@ -588,6 +623,32 @@ class SliderProperties extends CustomProperties with FieldsHolder {
       () => thumbStrokeWidth,
       (double value) => thumbStrokeWidth = value,
     ),
+    'thumbShape': EnumFieldAccess<SliderThumbShapeEnum>(
+      'Thumb Shape',
+      'Shape of slider\'s thumb: a circle or a rounded rectangle.',
+      () => thumbShape,
+      (SliderThumbShapeEnum value) => thumbShape = value,
+      defaultValue: () => SliderThumbShapeEnum.circle,
+      options: () => SliderThumbShapeEnum.values,
+    ),
+    'thumbWidth': NumFieldAccess<double>(
+      'Thumb Width',
+      'Width of slider\'s rectangular thumb.',
+      () => thumbWidth,
+      (double value) => thumbWidth = value,
+    ),
+    'thumbHeight': NumFieldAccess<double>(
+      'Thumb Height',
+      'Height of slider\'s rectangular thumb.',
+      () => thumbHeight,
+      (double value) => thumbHeight = value,
+    ),
+    'thumbCornerRadius': NumFieldAccess<double>(
+      'Thumb Corner Radius',
+      'Corner radius of slider\'s rectangular thumb.',
+      () => thumbCornerRadius,
+      (double value) => thumbCornerRadius = value,
+    ),
   };
 
   @override
@@ -611,6 +672,10 @@ class SliderProperties extends CustomProperties with FieldsHolder {
     valueIndicatorTextColor,
     valueIndicatorFontSize,
     thumbRadius,
+    thumbShape,
+    thumbWidth,
+    thumbHeight,
+    thumbCornerRadius,
     showThumb,
     trackShape,
     tickMarkRadius,
 
@@ -128,9 +128,24 @@ const String variablePathComposingPattern =
 /// Regex for [variablePathComposingPattern].
 final RegExp variablePathComposingRegex = RegExp(variablePathComposingPattern);
 
+const String _formulaAtom = r'''[^{}'"]|'[^']*'|"[^"]*"''';
+const String _formulaBody0 = '(?:$_formulaAtom)*';
+const String _formulaBody1 = '(?:$_formulaAtom|\\{$_formulaBody0\\})*';
+const String _formulaBody2 = '(?:$_formulaAtom|\\{$_formulaBody1\\})*';
+
+/// The expression of a `${= ...}` formula: text and quoted strings, with
+/// braces only in balanced pairs nested up to three deep, such as the object
+/// literal in `${= merge(r, {plays: r.plays + 1})}`. Braces inside quoted
+/// strings don't count.
+const String formulaExpressionPattern =
+    '(?:$_formulaAtom|\\{$_formulaBody2\\})*';
+
 /// A regex that matches any variable path identifier in a string that is
-/// wrapped in ${} curly braces.
-const String variableSyntaxIdentifierPattern = r'(?<!\\)\$\{(?<value>[^}]+)\}';
+/// wrapped in ${} curly braces. A formula (`${= ...}`) matches whole, braces
+/// of its object literals included, so `${= merge(r, {a: 1})}` is one match
+/// rather than ending at the first `}`.
+const String variableSyntaxIdentifierPattern =
+    '(?<!\\\\)\\\$\\{(?<value>=$formulaExpressionPattern|[^}]+)\\}';
 
 /// A regex that can match empty ${} syntax with partial match for curly braces.
 const String emptyVariableSyntaxIdentifierPattern = r'^(?<!\\)\${?}?$';