Fix text editor cursor display

examples/text_editor.zig

@@ -92,7 +92,7 @@ const Model = struct {
         const cursor_info = std.fmt.allocPrint(
             ctx.allocator,
             "Ln {d}, Col {d} | {d} lines",
-            .{ self.editor.cursor_row + 1, self.editor.cursor_col + 1, self.editor.lineCount() },
+            .{ self.editor.cursor_row + 1, self.editor.cursorDisplayColumn() + 1, self.editor.lineCount() },
         ) catch "";
 
         var status_style = zz.Style{};

src/components/text_area.zig

@@ -266,6 +266,7 @@ pub const TextArea = struct {
             if (self.lines.items.len >= max) return;
         }
 
+        self.clampCursorToLineBoundary();
         const line = self.currentLine();
         const rest = line.items[self.cursor_col..];
 
@@ -319,6 +320,7 @@ pub const TextArea = struct {
 
     fn deleteForward(self: *TextArea) void {
         const line = self.currentLine();
+        self.clampCursorToLineBoundary();
         if (self.cursor_col < line.items.len) {
             // Delete character at cursor
             const byte_len = std.unicode.utf8ByteSequenceLength(line.items[self.cursor_col]) catch 1;
@@ -336,11 +338,13 @@ pub const TextArea = struct {
     }
 
     fn killToEndOfLine(self: *TextArea) void {
+        self.clampCursorToLineBoundary();
         const line = self.currentLine();
         line.shrinkRetainingCapacity(self.cursor_col);
     }
 
     fn killToStartOfLine(self: *TextArea) void {
+        self.clampCursorToLineBoundary();
         const line = self.currentLine();
         std.mem.copyForwards(u8, line.items[0..], line.items[self.cursor_col..]);
         line.shrinkRetainingCapacity(line.items.len - self.cursor_col);
@@ -355,6 +359,7 @@ pub const TextArea = struct {
                 self.cursor_row = self.lines.items.len - 1;
             }
             self.cursor_col = @min(self.cursor_col, self.currentLine().items.len);
+            self.clampCursorToLineBoundary();
         } else {
             self.currentLine().clearRetainingCapacity();
             self.cursor_col = 0;
@@ -365,17 +370,20 @@ pub const TextArea = struct {
         if (self.cursor_row > 0) {
             self.cursor_row -= 1;
             self.cursor_col = @min(self.cursor_col, self.currentLine().items.len);
+            self.clampCursorToLineBoundary();
         }
     }
 
     fn moveCursorDown(self: *TextArea) void {
         if (self.cursor_row < self.lines.items.len - 1) {
             self.cursor_row += 1;
             self.cursor_col = @min(self.cursor_col, self.currentLine().items.len);
+            self.clampCursorToLineBoundary();
         }
     }
 
     fn moveCursorLeft(self: *TextArea) void {
+        self.clampCursorToLineBoundary();
         if (self.cursor_col > 0) {
             self.cursor_col -= 1;
             // Handle UTF-8 continuation bytes
@@ -391,6 +399,7 @@ pub const TextArea = struct {
 
     fn moveCursorRight(self: *TextArea) void {
         const line = self.currentLine();
+        self.clampCursorToLineBoundary();
         if (self.cursor_col < line.items.len) {
             const byte_len = std.unicode.utf8ByteSequenceLength(line.items[self.cursor_col]) catch 1;
             self.cursor_col = @min(self.cursor_col + byte_len, line.items.len);
@@ -407,6 +416,7 @@ pub const TextArea = struct {
             self.cursor_row = 0;
         }
         self.cursor_col = @min(self.cursor_col, self.currentLine().items.len);
+        self.clampCursorToLineBoundary();
     }
 
     fn pageDown(self: *TextArea) void {
@@ -416,6 +426,7 @@ pub const TextArea = struct {
             self.cursor_row = self.lines.items.len - 1;
         }
         self.cursor_col = @min(self.cursor_col, self.currentLine().items.len);
+        self.clampCursorToLineBoundary();
     }
 
     fn ensureVisible(self: *TextArea) void {
@@ -436,6 +447,11 @@ pub const TextArea = struct {
         }
     }
 
+    /// Cursor column in terminal cells (display width), 0-indexed.
+    pub fn cursorDisplayColumn(self: *const TextArea) usize {
+        return self.cursorDisplayCol();
+    }
+
     /// Render the text area
     pub fn view(self: *const TextArea, allocator: std.mem.Allocator) ![]const u8 {
         var result = std.array_list.Managed(u8).init(allocator);
@@ -567,4 +583,17 @@ pub const TextArea = struct {
         }
         return display_col;
     }
+
+    fn clampCursorToLineBoundary(self: *TextArea) void {
+        const line = self.currentLine();
+        self.cursor_col = clampToUtf8Boundary(line.items, self.cursor_col);
+    }
+
+    fn clampToUtf8Boundary(line: []const u8, pos: usize) usize {
+        var clamped = @min(pos, line.len);
+        while (clamped > 0 and clamped < line.len and (line[clamped] & 0xC0) == 0x80) {
+            clamped -= 1;
+        }
+        return clamped;
+    }
 };

tests/input_tests.zig

@@ -150,3 +150,31 @@ test "TextArea accepts multi-character paste commits (IME-like)" {
     defer testing.allocator.free(value);
     try testing.expectEqualStrings("中文输入", value);
 }
+
+test "TextArea keeps cursor on UTF-8 boundaries after vertical move" {
+    var area = zz.TextArea.init(testing.allocator);
+    defer area.deinit();
+
+    area.setValue("abcd\n中文") catch unreachable;
+    area.handleKey(.{ .key = .end });
+    area.handleKey(.{ .key = .down });
+
+    const value = try area.getValue(testing.allocator);
+    defer testing.allocator.free(value);
+    try testing.expectEqualStrings("abcd\n中文", value);
+
+    area.handleKey(.{ .key = .{ .char = 'X' } });
+    const updated = try area.getValue(testing.allocator);
+    defer testing.allocator.free(updated);
+    try testing.expectEqualStrings("abcd\n中X文", updated);
+}
+
+test "TextArea cursorDisplayColumn uses visual width for CJK" {
+    var area = zz.TextArea.init(testing.allocator);
+    defer area.deinit();
+
+    area.handleKey(.{ .key = .{ .paste = "中a" } });
+    area.handleKey(.{ .key = .left });
+
+    try testing.expectEqual(@as(usize, 2), area.cursorDisplayColumn());
+}