fix(core): robust syntax highlight — dynamic positioning, mode-aware, proper toggle

- _positionHighlightLayer(): set left offset dynamically from gutter width
- Use hidden attribute instead of clearing innerHTML for toggle
- Skip highlight in preview mode
- Initialize highlightLayer hidden state on construction
- Remove hardcoded left:36px CSS

src/core.js

@@ -233,6 +233,9 @@ class MarkdownEditor {
     if (this.config.syntaxHighlight) {
       textarea.classList.add('me-textarea-highlight');
     }
+    if (highlightLayer && !this.config.syntaxHighlight) {
+      highlightLayer.hidden = true;
+    }
     editorInner.appendChild(textarea);
 
     editorPane.appendChild(editorInner);
@@ -1329,12 +1332,13 @@ class MarkdownEditor {
   }
 
   _doHighlight() {
+    if (!this.highlightLayer || this.highlightLayer.hidden || this._mode === 'preview') return;
     const text = this._value || '';
     if (!text) { this.highlightLayer.innerHTML = ''; return; }
-    // 超过 50KB 跳过，避免大文档卡顿
     if (text.length > 50000) { this.highlightLayer.innerHTML = escapeHTML(text) + '\n'; return; }
 
     let html = escapeHTML(text);
+    this._positionHighlightLayer();
 
     // 正则按优先级：先匹配更具体的模式，避免被泛模式吞掉
     // 1. 图片（先于链接）
@@ -1370,13 +1374,27 @@ class MarkdownEditor {
     if (this.textarea) {
       this.textarea.classList.toggle('me-textarea-highlight', this.config.syntaxHighlight);
     }
-    if (!this.config.syntaxHighlight && this.highlightLayer) {
-      this.highlightLayer.innerHTML = '';
+    if (this.highlightLayer) {
+      this.highlightLayer.hidden = !this.config.syntaxHighlight;
+    }
+    if (this.config.syntaxHighlight) {
+      this._positionHighlightLayer();
+      this._doHighlight();
     }
-    if (this.config.syntaxHighlight) this._doHighlight();
     return this;
   }
 
+  _positionHighlightLayer() {
+    if (!this.highlightLayer) return;
+    // 动态设置左边距：有行号时对齐行号宽度，无行号时从 0 开始
+    if (this.gutter && this.config.lineNumbers && this.gutter.style.display !== 'none') {
+      const w = this.gutter.getBoundingClientRect().width;
+      this.highlightLayer.style.left = w + 'px';
+    } else {
+      this.highlightLayer.style.left = '0';
+    }
+  }
+
   _updateWordCount() {
     if (!this.config.wordCount || !this.statusEl) return;
     const text = this._value || '';

src/styles.js

@@ -134,13 +134,15 @@ const generateCSS = () => {
     .me-editor-inner { display: flex; height: 100%; overflow: hidden; position: relative; }
     /* 语法高亮叠加层（v0.1.14） */
     .me-highlight-layer {
-      position: absolute; top: 0; left: 36px; right: 0; bottom: 0;
+      position: absolute; top: 0; right: 0; bottom: 0;
       padding: 16px 18px; margin: 0; border: 0; outline: 0; overflow: hidden;
       font-family: var(--md-mono); font-size: 13.5px; line-height: 1.7;
       white-space: pre-wrap; word-wrap: break-word;
       pointer-events: none; background: transparent;
-      color: var(--md-text);
+      color: var(--md-text); z-index: 0;
     }
+    /* 隐藏高亮层（语法高亮关闭时） */
+    .me-highlight-layer[hidden] { display: none; }
     .mh-heading { color: var(--md-accent); font-weight: 700; }
     .mh-bold { color: var(--md-text); font-weight: 700; }
     .mh-italic { font-style: italic; }