fix(core): fix gutter scroll desync with textarea content

- Match gutter font-size to textarea (13.5px) so line heights align
- Always sync gutter scrollTop regardless of mode
- Sync gutter scroll position after undo/redo history apply

src/core.js

@@ -351,11 +351,15 @@ class MarkdownEditor {
     ta.addEventListener('keyup', onCursorActivity);
 
     const onScroll = () => {
-      // 同步行号滚动
-      if (this.gutter) this.gutter.scrollTop = ta.scrollTop;
+      // 同步行号滚动（始终同步，不限于分屏模式）
+      if (this.gutter) {
+        this.gutter.scrollTop = ta.scrollTop;
+      }
+      // 同步预览滚动（仅分屏模式）
       if (!this.config.syncScroll || this._mode !== 'split') return;
       const max = ta.scrollHeight - ta.clientHeight;
-      const ratio = max > 0 ? ta.scrollTop / max : 0;
+      if (max <= 0) return;
+      const ratio = ta.scrollTop / max;
       const pmax = this.previewPane.scrollHeight - this.previewPane.clientHeight;
       this.previewPane.scrollTop = ratio * pmax;
     };
@@ -870,6 +874,8 @@ class MarkdownEditor {
     this._render();
     this._renderGutter();
     this._updateWordCount();
+    // 同步行号滚动位置
+    if (this.gutter) this.gutter.scrollTop = this.textarea.scrollTop;
     this._emit('change', this._value);
     if (typeof this.config.onChange === 'function') {
       try { this.config.onChange(this._value, this); } catch (e) { console.error(e); }

src/styles.js

@@ -140,7 +140,7 @@ const generateCSS = () => {
       border-right: 1px solid var(--md-border);
       color: var(--md-muted);
       font-family: var(--md-mono);
-      font-size: 12px;
+      font-size: 13.5px;
       line-height: 1.7;
       text-align: right;
       user-select: none;