feat: Add vertical centering for operators and relations

lib/src/ast/nodes/symbol.dart

@@ -103,7 +103,9 @@ class SymbolNode extends LeafNode {
       oldOptions.color != newOptions.color ||
       oldOptions.mathFontOptions != newOptions.mathFontOptions ||
       oldOptions.textFontOptions != newOptions.textFontOptions ||
-      oldOptions.sizeMultiplier != newOptions.sizeMultiplier;
+      oldOptions.sizeMultiplier != newOptions.sizeMultiplier ||
+      oldOptions.centerOperators != newOptions.centerOperators ||
+      oldOptions.forceVariableBaseline != newOptions.forceVariableBaseline;
 
   @override
   AtomType get leftType => atomType;

lib/src/ast/options.dart

@@ -70,6 +70,16 @@ class MathOptions {
   /// {@endtemplate}
   final double logicalPpi;
 
+  /// Whether to vertically center binary operators and relations relative to
+  /// numbers. When true, operators like +, -, =, <, > will be visually centered
+  /// with the middle of numbers instead of sitting at the baseline.
+  final bool centerOperators;
+
+  /// When true, forces variables to stay at the baseline (not centered).
+  /// This is used for variables adjacent to numbers like "3x" where x should
+  /// share the baseline with 3.
+  final bool forceVariableBaseline;
+
   MathOptions._({
     required this.fontSize,
     required this.logicalPpi,
@@ -78,6 +88,8 @@ class MathOptions {
     this.sizeUnderTextStyle = MathSize.normalsize,
     this.textFontOptions,
     this.mathFontOptions,
+    this.centerOperators = false,
+    this.forceVariableBaseline = false,
     // required this.maxSize,
     // required this.minRuleThickness,
   });
@@ -97,6 +109,7 @@ class MathOptions {
     FontOptions? mathFontOptions,
     double? fontSize,
     double? logicalPpi,
+    bool centerOperators = false,
     // required this.maxSize,
     // required this.minRuleThickness,
   }) {
@@ -114,6 +127,7 @@ class MathOptions {
       sizeUnderTextStyle: sizeUnderTextStyle,
       mathFontOptions: mathFontOptions,
       textFontOptions: textFontOptions,
+      centerOperators: centerOperators,
     );
   }
 
@@ -233,6 +247,8 @@ class MathOptions {
     MathSize? sizeUnderTextStyle,
     FontOptions? textFontOptions,
     FontOptions? mathFontOptions,
+    bool? centerOperators,
+    bool? forceVariableBaseline,
     // double maxSize,
     // num minRuleThickness,
   }) =>
@@ -244,6 +260,8 @@ class MathOptions {
         sizeUnderTextStyle: sizeUnderTextStyle ?? this.sizeUnderTextStyle,
         textFontOptions: textFontOptions ?? this.textFontOptions,
         mathFontOptions: mathFontOptions ?? this.mathFontOptions,
+        centerOperators: centerOperators ?? this.centerOperators,
+        forceVariableBaseline: forceVariableBaseline ?? this.forceVariableBaseline,
         // maxSize: maxSize ?? this.maxSize,
         // minRuleThickness: minRuleThickness ?? this.minRuleThickness,
       );

lib/src/ast/syntax_tree.dart

@@ -17,6 +17,7 @@ import '../widgets/mode.dart';
 import '../widgets/selectable.dart';
 import 'nodes/space.dart';
 import 'nodes/sqrt.dart';
+import 'nodes/symbol.dart';
 import 'options.dart';
 import 'spacing.dart';
 import 'types.dart';
@@ -646,8 +647,69 @@ class EquationRowNode extends ParentableNode<GreenNode>
   }
 
   @override
-  List<MathOptions> computeChildOptions(MathOptions options) =>
-      List.filled(children.length, options, growable: false);
+  List<MathOptions> computeChildOptions(MathOptions options) {
+    // Check each child to determine if it's a variable adjacent to a number.
+    // Variables adjacent to numbers (like "3x") should stay at baseline.
+    // Variables not adjacent to numbers should be centered.
+    return List.generate(children.length, (index) {
+      final child = children[index];
+
+      // Only process SymbolNode children
+      if (child is! SymbolNode) {
+        return options;
+      }
+
+      // Check if this is an ordinary character (variable/number)
+      if (child.atomType != AtomType.ord) {
+        return options;
+      }
+
+      // Check if this is a digit (numbers always stay at baseline)
+      final symbol = child.symbol;
+      if (_isDigit(symbol)) {
+        return options;
+      }
+
+      // This is a variable (letter). Check if it's adjacent to a number.
+      final isAdjacentToNumber = _isAdjacentToNumber(index);
+
+      if (isAdjacentToNumber) {
+        // Variable is adjacent to number - force baseline
+        return options.copyWith(forceVariableBaseline: true);
+      }
+
+      // Variable is not adjacent to number - will be centered
+      return options;
+    }, growable: false);
+  }
+
+  /// Check if character at index is adjacent to a number (no operator between)
+  bool _isAdjacentToNumber(int index) {
+    // Check previous sibling
+    if (index > 0) {
+      final prev = children[index - 1];
+      if (prev is SymbolNode && _isDigit(prev.symbol)) {
+        return true;
+      }
+    }
+
+    // Check next sibling
+    if (index < children.length - 1) {
+      final next = children[index + 1];
+      if (next is SymbolNode && _isDigit(next.symbol)) {
+        return true;
+      }
+    }
+
+    return false;
+  }
+
+  /// Check if a symbol is a digit (0-9)
+  static bool _isDigit(String symbol) {
+    if (symbol.isEmpty) return false;
+    final code = symbol.codeUnitAt(0);
+    return code >= 0x30 && code <= 0x39; // '0' to '9'
+  }
 
   @override
   bool shouldRebuildWidget(MathOptions oldOptions, MathOptions newOptions) =>

lib/src/render/layout/reset_dimension.dart

@@ -8,13 +8,15 @@ class ResetDimension extends SingleChildRenderObjectWidget {
   final double? height;
   final double? depth;
   final double? width;
+  final double verticalOffset;
   final CrossAxisAlignment horizontalAlignment;
 
   const ResetDimension({
     Key? key,
     this.height,
     this.depth,
     this.width,
+    this.verticalOffset = 0.0,
     this.horizontalAlignment = CrossAxisAlignment.center,
     required Widget child,
   }) : super(key: key, child: child);
@@ -25,6 +27,7 @@ class ResetDimension extends SingleChildRenderObjectWidget {
         layoutHeight: height,
         layoutWidth: width,
         layoutDepth: depth,
+        verticalOffset: verticalOffset,
         horizontalAlignment: horizontalAlignment,
       );
 
@@ -35,6 +38,7 @@ class ResetDimension extends SingleChildRenderObjectWidget {
         ..layoutHeight = height
         ..layoutDepth = depth
         ..layoutWidth = width
+        ..verticalOffset = verticalOffset
         ..horizontalAlignment = horizontalAlignment;
 }
 
@@ -44,10 +48,12 @@ class RenderResetDimension extends RenderShiftedBox {
     double? layoutHeight,
     double? layoutDepth,
     double? layoutWidth,
+    double verticalOffset = 0.0,
     CrossAxisAlignment horizontalAlignment = CrossAxisAlignment.center,
   })  : _layoutHeight = layoutHeight,
         _layoutDepth = layoutDepth,
         _layoutWidth = layoutWidth,
+        _verticalOffset = verticalOffset,
         _horizontalAlignment = horizontalAlignment,
         super(child);
 
@@ -78,6 +84,15 @@ class RenderResetDimension extends RenderShiftedBox {
     }
   }
 
+  double get verticalOffset => _verticalOffset;
+  double _verticalOffset;
+  set verticalOffset(double value) {
+    if (_verticalOffset != value) {
+      _verticalOffset = value;
+      markNeedsLayout();
+    }
+  }
+
   CrossAxisAlignment get horizontalAlignment => _horizontalAlignment;
   CrossAxisAlignment _horizontalAlignment;
   set horizontalAlignment(CrossAxisAlignment value) {
@@ -162,7 +177,9 @@ class RenderResetDimension extends RenderShiftedBox {
     }
 
     if (!dry) {
-      child.offset = Offset(dx, height - childHeight);
+      // Apply vertical offset after baseline alignment
+      // Negative offset shifts up, positive shifts down
+      child.offset = Offset(dx, height - childHeight - verticalOffset);
     }
 
     return Size(width, height + depth);

lib/src/render/symbols/make_symbol.dart

@@ -1,4 +1,3 @@
-
 import 'package:flutter/widgets.dart';
 
 import '../../ast/options.dart';
@@ -10,6 +9,7 @@ import '../../ast/syntax_tree.dart';
 import '../../ast/types.dart';
 import '../../font/metrics/font_metrics.dart';
 import '../layout/reset_dimension.dart';
+import '../utils/alignment_utils.dart';
 import 'make_composite.dart';
 
 BuildResult makeBaseSymbol({
@@ -61,7 +61,7 @@ BuildResult makeBaseSymbol({
             italic: italic,
             skew: charMetrics.skew.cssEm.toLpUnder(options),
             widget: makeChar(symbol, font, charMetrics, options,
-                needItalic: mode == Mode.math),
+                needItalic: mode == Mode.math, atomType: atomType),
           );
         } else if (ligatures.containsKey(symbol) &&
             font.fontFamily == 'Typewriter') {
@@ -73,8 +73,8 @@ BuildResult makeBaseSymbol({
               crossAxisAlignment: CrossAxisAlignment.baseline,
               textBaseline: TextBaseline.alphabetic,
               children: expandedText
-                  .map((e) =>
-                      makeChar(e, font!, lookupChar(e, font, mode), options))
+                  .map((e) => makeChar(e, font!, lookupChar(e, font, mode),
+                      options, atomType: atomType))
                   .toList(growable: false),
             ),
             italic: 0.0,
@@ -97,7 +97,7 @@ BuildResult makeBaseSymbol({
     return BuildResult(
       options: options,
       widget: makeChar(char, defaultFont, characterMetrics, options,
-          needItalic: mode == Mode.math),
+          needItalic: mode == Mode.math, atomType: atomType),
       italic: italic,
       skew: characterMetrics?.skew.cssEm.toLpUnder(options) ?? 0.0,
     );
@@ -123,16 +123,67 @@ BuildResult makeBaseSymbol({
     italic: 0.0,
     skew: 0.0,
     widget: makeChar(symbol, const FontOptions(), null, options,
-        needItalic: mode == Mode.math),
+        needItalic: mode == Mode.math, atomType: atomType),
   );
 }
 
-Widget makeChar(String character, FontOptions font,
-    CharacterMetrics? characterMetrics, MathOptions options,
-    {bool needItalic = false}) {
+Widget makeChar(
+  String character,
+  FontOptions font,
+  CharacterMetrics? characterMetrics,
+  MathOptions options, {
+  bool needItalic = false,
+  AtomType? atomType,
+}) {
+  // Calculate vertical offset for visual centering
+  double verticalOffset = 0.0;
+
+  if (options.centerOperators &&
+      characterMetrics != null &&
+      atomType != null) {
+    final height = characterMetrics.height;
+    final depth = characterMetrics.depth;
+
+    // Centering rules:
+    // 1. Numbers: Always at baseline (tall, serve as reference)
+    // 2. Operators (+, -, =, etc.): Always centered at math axis
+    // 3. Variables (x, y, a, b):
+    //    - If adjacent to number (like "3x"): at baseline (forceVariableBaseline=true)
+    //    - If standalone (like "x" in "3 + 5 = x"): centered
+    //    - If with other variables (like "xy"): centered
+    //
+    // This makes expressions look balanced while keeping "3x" style
+    // coefficients properly aligned.
+
+    final isOperator = atomType == AtomType.bin || atomType == AtomType.rel;
+    final isVariable = atomType == AtomType.ord && 
+        AlignmentConstants.shouldApplyCentering(height);
+
+    // Center operators always, and center variables unless forced to baseline
+    final shouldCenter = AlignmentConstants.shouldApplyCentering(height) &&
+        (isOperator || (isVariable && !options.forceVariableBaseline));
+
+    if (shouldCenter) {
+      // Calculate offset in em units, then convert to logical pixels
+      final offsetEm = AlignmentConstants.calculateAlignmentOffset(
+        charHeight: height,
+        charDepth: depth,
+      );
+      verticalOffset = offsetEm.cssEm.toLpUnder(options);
+    }
+  }
+
+  // When centering is applied, report the number height as the baseline
+  // so that all characters in a line contribute the same baseline,
+  // ensuring consistent alignment between formulas in Quill.
+  final reportedHeight = (verticalOffset != 0.0)
+      ? AlignmentConstants.numberHeight.cssEm.toLpUnder(options)
+      : characterMetrics?.height.cssEm.toLpUnder(options);
+
   final charWidget = ResetDimension(
-    height: characterMetrics?.height.cssEm.toLpUnder(options),
+    height: reportedHeight,
     depth: characterMetrics?.depth.cssEm.toLpUnder(options),
+    verticalOffset: verticalOffset,
     child: RichText(
       text: TextSpan(
         text: character,