schuyler · yusufm · May 3, 2026
diff --git a/MacDown/Code/Document/MPDocument.m b/MacDown/Code/Document/MPDocument.m
@@ -75,6 +75,8 @@
             @"editorBaseFontInfo", @"extensionFootnotes",
             @"editorHorizontalInset", @"editorVerticalInset",
             @"editorWidthLimited", @"editorMaximumWidth", @"editorLineSpacing",
+            @"editorColumnGuideEnabled", @"editorColumnGuideWidth",
+            @"editorWrapsAtColumnGuide",
             @"editorOnRight", @"editorStyleName", @"editorShowWordCount",
             @"editorScrollsPastEnd",
             @"htmlMathJax", @"htmlMathJaxInlineDollar", nil
@@ -267,6 +269,7 @@ - (void)refreshHeaderCacheAfterResize;
 - (void)windowDidEndLiveResize:(NSNotification *)notification;
 - (void)windowDidChangeFullScreen:(NSNotification *)notification;
 - (void)applyEditorStartInPreviewModePreference;
+- (void)applyColumnGuidePreferences;
 // Commit 8 (gap 9): MathJax generation counter accessor (used by tests via category)
 - (NSUInteger)mathJaxRenderGeneration;
 
@@ -899,6 +902,39 @@ - (BOOL)validateUserInterfaceItem:(id<NSValidatedUserInterfaceItem>)item
             return NO;
         }
     }
+    else if (action == @selector(toggleColumnGuide:))
+    {
+        ((NSMenuItem *)item).state = self.preferences.editorColumnGuideEnabled
+            ? NSControlStateValueOn : NSControlStateValueOff;
+        return self.editor != nil;
+    }
+    else if (action == @selector(toggleWrapAtColumnGuide:))
+    {
+        ((NSMenuItem *)item).state = self.preferences.editorWrapsAtColumnGuide
+            ? NSControlStateValueOn : NSControlStateValueOff;
+        return self.editor != nil;
+    }
+    else if (action == @selector(setColumnGuideWidth80:))
+    {
+        ((NSMenuItem *)item).state =
+            self.preferences.editorColumnGuideWidth == 80
+                ? NSControlStateValueOn : NSControlStateValueOff;
+        return self.editor != nil;
+    }
+    else if (action == @selector(setColumnGuideWidth100:))
+    {
+        ((NSMenuItem *)item).state =
+            self.preferences.editorColumnGuideWidth == 100
+                ? NSControlStateValueOn : NSControlStateValueOff;
+        return self.editor != nil;
+    }
+    else if (action == @selector(setColumnGuideWidth120:))
+    {
+        ((NSMenuItem *)item).state =
+            self.preferences.editorColumnGuideWidth == 120
+                ? NSControlStateValueOn : NSControlStateValueOff;
+        return self.editor != nil;
+    }
     return result;
 }
 
@@ -1995,6 +2031,33 @@ - (IBAction)toggleEditorPane:(id)sender
     [self toggleSplitterCollapsingEditorPane:YES];
 }
 
+- (IBAction)toggleColumnGuide:(id)sender
+{
+    self.preferences.editorColumnGuideEnabled =
+        !self.preferences.editorColumnGuideEnabled;
+}
+
+- (IBAction)toggleWrapAtColumnGuide:(id)sender
+{
+    self.preferences.editorWrapsAtColumnGuide =
+        !self.preferences.editorWrapsAtColumnGuide;
+}
+
+- (IBAction)setColumnGuideWidth80:(id)sender
+{
+    self.preferences.editorColumnGuideWidth = 80;
+}
+
+- (IBAction)setColumnGuideWidth100:(id)sender
+{
+    self.preferences.editorColumnGuideWidth = 100;
+}
+
+- (IBAction)setColumnGuideWidth120:(id)sender
+{
+    self.preferences.editorColumnGuideWidth = 120;
+}
+
 - (IBAction)render:(id)sender
 {
     [self.renderer parseAndRenderLater];
@@ -2152,14 +2215,19 @@ - (void)setupEditor:(NSString *)changedKey
     if (!changedKey || [changedKey isEqualToString:@"editorHorizontalInset"]
             || [changedKey isEqualToString:@"editorVerticalInset"]
             || [changedKey isEqualToString:@"editorWidthLimited"]
-            || [changedKey isEqualToString:@"editorMaximumWidth"])
+            || [changedKey isEqualToString:@"editorMaximumWidth"]
+            || [changedKey isEqualToString:@"editorColumnGuideEnabled"]
+            || [changedKey isEqualToString:@"editorColumnGuideWidth"]
+            || [changedKey isEqualToString:@"editorWrapsAtColumnGuide"])
     {
         [self adjustEditorInsets];
+        [self applyColumnGuidePreferences];
     }
 
     if (!changedKey || [changedKey isEqualToString:@"editorBaseFontInfo"]
             || [changedKey isEqualToString:@"editorStyleName"]
-            || [changedKey isEqualToString:@"editorLineSpacing"])
+            || [changedKey isEqualToString:@"editorLineSpacing"]
+            || [changedKey isEqualToString:@"editorColumnGuideWidth"])
     {
         NSMutableParagraphStyle *style = [[NSMutableParagraphStyle alloc] init];
         style.lineSpacing = self.preferences.editorLineSpacing;
@@ -2209,6 +2277,7 @@ - (void)setupEditor:(NSString *)changedKey
         if (backgroundCGColor)
             layer.backgroundColor = backgroundCGColor;
         self.editorContainer.layer = layer;
+        [self applyColumnGuidePreferences];
     }
 
     if ([changedKey isEqualToString:@"editorBaseFontInfo"])
@@ -2298,6 +2367,43 @@ - (void)adjustEditorInsets
     self.editor.textContainerInset = NSMakeSize(x, y);
 }
 
+- (void)applyColumnGuidePreferences
+{
+    NSInteger column = self.preferences.editorColumnGuideWidth;
+    if (column <= 0)
+        column = 80;
+
+    self.editor.columnGuideVisible =
+        self.preferences.editorColumnGuideEnabled;
+    self.editor.columnGuideColumn = column;
+    self.editor.wrapsAtColumnGuide =
+        self.preferences.editorWrapsAtColumnGuide;
+
+    NSTextContainer *container = self.editor.textContainer;
+    if (!container)
+        return;
+
+    if (self.editor.wrapsAtColumnGuide)
+    {
+        NSFont *font = self.editor.font ?:
+            self.preferences.editorBaseFont ?:
+            [NSFont userFixedPitchFontOfSize:0.0];
+        NSDictionary *attributes = @{NSFontAttributeName: font};
+        CGFloat characterWidth = [@"0" sizeWithAttributes:attributes].width;
+        CGFloat width = characterWidth * column +
+            2.0 * container.lineFragmentPadding;
+        container.widthTracksTextView = NO;
+        container.containerSize = NSMakeSize(width, CGFLOAT_MAX);
+    }
+    else
+    {
+        container.widthTracksTextView = YES;
+        container.containerSize = NSMakeSize(CGFLOAT_MAX, CGFLOAT_MAX);
+    }
+
+    self.editor.needsDisplay = YES;
+}
+
 - (void)redrawDivider
 {
     if (!self.editorVisible)

diff --git a/MacDown/Code/Preferences/MPPreferences.h b/MacDown/Code/Preferences/MPPreferences.h
@@ -48,6 +48,9 @@ extern NSString * const MPDidDetectFreshInstallationNotification;
 @property (assign) CGFloat editorLineSpacing;
 @property (assign) BOOL editorWidthLimited;
 @property (assign) CGFloat editorMaximumWidth;
+@property (assign) BOOL editorColumnGuideEnabled;
+@property (assign) NSInteger editorColumnGuideWidth;
+@property (assign) BOOL editorWrapsAtColumnGuide;
 @property (assign) BOOL editorOnRight;
 @property (assign) BOOL editorStartInPreviewMode;
 @property (assign) BOOL editorShowWordCount;

diff --git a/MacDown/Code/Preferences/MPPreferences.m b/MacDown/Code/Preferences/MPPreferences.m
@@ -247,6 +247,9 @@ - (void)migratePreferencesFromLegacyBundleIdentifierIfNeeded
 @dynamic editorLineSpacing;
 @dynamic editorWidthLimited;
 @dynamic editorMaximumWidth;
+@dynamic editorColumnGuideEnabled;
+@dynamic editorColumnGuideWidth;
+@dynamic editorWrapsAtColumnGuide;
 @dynamic editorOnRight;
 @dynamic editorStartInPreviewMode;
 @dynamic editorShowWordCount;
@@ -427,6 +430,8 @@ - (void)loadDefaultUserDefaults
     NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
     if (![defaults objectForKey:@"editorMaximumWidth"])
         self.editorMaximumWidth = 1000.0;
+    if (![defaults objectForKey:@"editorColumnGuideWidth"])
+        self.editorColumnGuideWidth = 80;
     if (![defaults objectForKey:@"editorAutoIncrementNumberedLists"])
         self.editorAutoIncrementNumberedLists = YES;
     if (![defaults objectForKey:@"editorInsertPrefixInBlock"])

diff --git a/MacDown/Code/View/MPEditorView.h b/MacDown/Code/View/MPEditorView.h
@@ -11,7 +11,11 @@
 @interface MPEditorView : NSTextView
 
 @property BOOL scrollsPastEnd;
+@property BOOL columnGuideVisible;
+@property NSInteger columnGuideColumn;
+@property BOOL wrapsAtColumnGuide;
 - (NSRect)contentRect;
+- (CGFloat)columnGuideXPosition;
 - (void)paste:(id)sender;
 
 @end
diff --git a/MacDown/Code/View/MPEditorView.m b/MacDown/Code/View/MPEditorView.m
@@ -38,13 +38,35 @@ @implementation MPEditorView
 @synthesize contentRect = _contentRect;
 @synthesize scrollsPastEnd = _scrollsPastEnd;
 
+- (void)setColumnGuideVisible:(BOOL)columnGuideVisible
+{
+    _columnGuideVisible = columnGuideVisible;
+    self.needsDisplay = YES;
+}
+
+- (void)setColumnGuideColumn:(NSInteger)columnGuideColumn
+{
+    _columnGuideColumn = MAX(1, columnGuideColumn);
+    self.needsDisplay = YES;
+}
+
 - (BOOL)scrollsPastEnd
 {
     @synchronized(self) {
         return _scrollsPastEnd;
     }
 }
 
+- (CGFloat)columnGuideXPosition
+{
+    NSFont *font = self.font ?: [NSFont userFixedPitchFontOfSize:0.0];
+    NSDictionary *attributes = @{NSFontAttributeName: font};
+    CGFloat characterWidth = [@"0" sizeWithAttributes:attributes].width;
+    CGFloat padding = self.textContainer.lineFragmentPadding;
+    return self.textContainerOrigin.x + padding +
+        characterWidth * MAX(1, self.columnGuideColumn);
+}
+
 - (void)awakeFromNib {
     [self registerForDraggedTypes:@[NSPasteboardTypeFileURL]];
     [super awakeFromNib];
@@ -168,6 +190,25 @@ - (void)didChangeText
         [self updateContentGeometry];
 }
 
+- (void)drawViewBackgroundInRect:(NSRect)rect
+{
+    [super drawViewBackgroundInRect:rect];
+
+    if (!self.columnGuideVisible || self.columnGuideColumn <= 0)
+        return;
+
+    CGFloat x = floor([self columnGuideXPosition]) + 0.5;
+    if (x < NSMinX(rect) || x > NSMaxX(rect))
+        return;
+
+    NSBezierPath *path = [NSBezierPath bezierPath];
+    [path moveToPoint:NSMakePoint(x, NSMinY(rect))];
+    [path lineToPoint:NSMakePoint(x, NSMaxY(rect))];
+    [[NSColor separatorColor] setStroke];
+    path.lineWidth = 1.0;
+    [path stroke];
+}
+
 /** Overridden to advertise markdown UTType support for pasteboard operations.
  */
 - (NSArray<NSPasteboardType> *)writablePasteboardTypes

diff --git a/MacDown/Localization/Base.lproj/MainMenu.xib b/MacDown/Localization/Base.lproj/MainMenu.xib
@@ -367,6 +367,44 @@
                                     <binding destination="LEY-zf-22t" name="enabled" keyPath="self.currentDocument.previewVisible" id="5e7-rJ-oX2"/>
                                 </connections>
                             </menuItem>
+                            <menuItem title="Column Guide" id="K8f-gQ-VwD">
+                                <modifierMask key="keyEquivalentModifierMask"/>
+                                <menu key="submenu" title="Column Guide" id="v6J-FR-3yq">
+                                    <items>
+                                        <menuItem title="Show Column Guide" id="f4Z-W1-vnP">
+                                            <modifierMask key="keyEquivalentModifierMask"/>
+                                            <connections>
+                                                <action selector="toggleColumnGuide:" target="-1" id="5P6-eg-Xqa"/>
+                                            </connections>
+                                        </menuItem>
+                                        <menuItem title="Wrap at Column Guide" id="Ujc-WO-zaz">
+                                            <modifierMask key="keyEquivalentModifierMask"/>
+                                            <connections>
+                                                <action selector="toggleWrapAtColumnGuide:" target="-1" id="mzd-N9-YqB"/>
+                                            </connections>
+                                        </menuItem>
+                                        <menuItem isSeparatorItem="YES" id="v7y-Ag-a51"/>
+                                        <menuItem title="80 Columns" id="yvz-tM-mAM">
+                                            <modifierMask key="keyEquivalentModifierMask"/>
+                                            <connections>
+                                                <action selector="setColumnGuideWidth80:" target="-1" id="YAc-DN-hJD"/>
+                                            </connections>
+                                        </menuItem>
+                                        <menuItem title="100 Columns" id="NDe-1A-hS0">
+                                            <modifierMask key="keyEquivalentModifierMask"/>
+                                            <connections>
+                                                <action selector="setColumnGuideWidth100:" target="-1" id="ypl-hW-kv9"/>
+                                            </connections>
+                                        </menuItem>
+                                        <menuItem title="120 Columns" id="WXW-t8-dCa">
+                                            <modifierMask key="keyEquivalentModifierMask"/>
+                                            <connections>
+                                                <action selector="setColumnGuideWidth120:" target="-1" id="OaW-Jw-spo"/>
+                                            </connections>
+                                        </menuItem>
+                                    </items>
+                                </menu>
+                            </menuItem>
                             <menuItem isSeparatorItem="YES" id="onf-Mq-Pah"/>
                             <menuItem title="Left 1:3 Right" id="VW7-VH-9yl">
                                 <modifierMask key="keyEquivalentModifierMask"/>

diff --git a/MacDownTests/MPEditorViewSubstitutionTests.m b/MacDownTests/MPEditorViewSubstitutionTests.m
@@ -146,6 +146,33 @@ - (void)removePreferenceForKey:(NSString *)key
     [defaults synchronize];
 }
 
+#pragma mark - Column Guide Tests
+
+- (void)testColumnGuideXPositionUsesConfiguredColumn
+{
+    NSFont *font = [NSFont fontWithName:@"Menlo-Regular" size:12.0];
+    self.editorView.font = font;
+    self.editorView.textContainer.lineFragmentPadding = 0.0;
+    self.editorView.textContainerInset = NSZeroSize;
+    self.editorView.columnGuideColumn = 10;
+
+    CGFloat characterWidth =
+        [@"0" sizeWithAttributes:@{NSFontAttributeName: font}].width;
+
+    XCTAssertEqualWithAccuracy([self.editorView columnGuideXPosition],
+                               characterWidth * 10.0,
+                               0.5,
+                               @"Guide should be placed after the configured column");
+}
+
+- (void)testColumnGuideColumnClampsToPositiveValue
+{
+    self.editorView.columnGuideColumn = 0;
+
+    XCTAssertEqual(self.editorView.columnGuideColumn, 1,
+                   @"Column guide should clamp non-positive widths");
+}
+
 #pragma mark - Automatic Dash Substitution Tests
 
 /**

diff --git a/MacDownTests/MPPreferencesTests.m b/MacDownTests/MPPreferencesTests.m
@@ -152,6 +152,30 @@ - (void)testStartInPreviewModeToggle
     [self.preferences synchronize];
 }
 
+- (void)testColumnGuidePreferences
+{
+    BOOL originalEnabled = self.preferences.editorColumnGuideEnabled;
+    BOOL originalWrap = self.preferences.editorWrapsAtColumnGuide;
+    NSInteger originalWidth = self.preferences.editorColumnGuideWidth;
+
+    self.preferences.editorColumnGuideEnabled = YES;
+    self.preferences.editorWrapsAtColumnGuide = YES;
+    self.preferences.editorColumnGuideWidth = 100;
+    [self.preferences synchronize];
+
+    XCTAssertTrue(self.preferences.editorColumnGuideEnabled,
+                  @"Column guide should be ON");
+    XCTAssertTrue(self.preferences.editorWrapsAtColumnGuide,
+                  @"Column wrapping should be ON");
+    XCTAssertEqual(self.preferences.editorColumnGuideWidth, 100,
+                   @"Column guide width should persist");
+
+    self.preferences.editorColumnGuideEnabled = originalEnabled;
+    self.preferences.editorWrapsAtColumnGuide = originalWrap;
+    self.preferences.editorColumnGuideWidth = originalWidth;
+    [self.preferences synchronize];
+}
+
 - (void)testExtensionFlags
 {
     // Save originals