fix(core): scroll sync v2 — use scrollTop instead of selectionStart

Root cause: _syncScrollByHeading used cursor position (selectionStart)
to find nearest heading, but cursor doesn't move during scroll. Preview
was stuck at the same heading regardless of scroll position.

Fix: base heading detection on estimated viewport top line from scrollTop
+ lineHeight. Always run proportional sync first for smooth scrolling,
then fine-tune to nearest heading at viewport top.

src/core.js

@@ -1243,37 +1243,39 @@ class MarkdownEditor {
     const ta = this.textarea;
     if (!ta) return;
 
-    // 找到光标所在位置最近的标题
-    const pos = ta.selectionStart;
-    const before = this._value.substring(0, pos);
-    const headings = [...before.matchAll(/^(#{1,6})\s+(.+)$/gm)];
-    if (!headings.length) {
-      // 无标题，回退到比例同步
+    // 1. 先做比例同步（基础行为，保证平滑）
     const max = ta.scrollHeight - ta.clientHeight;
     if (max <= 0) return;
     const ratio = ta.scrollTop / max;
     const pmax = this.previewPane.scrollHeight - this.previewPane.clientHeight;
     this.previewPane.scrollTop = ratio * pmax;
-      return;
-    }
 
-    // 取最后一个标题（光标之前最近的标题）
+    // 2. 如果有标题，微调到最近的标题位置
+    const val = this._value;
+    if (!val) return;
+
+    // 估计 textarea 视口顶部的行号
+    const lineHeight = 13.5 * 1.7; // font-size * line-height
+    const scrollLines = Math.floor(ta.scrollTop / lineHeight);
+    const beforeLines = val.split('\n').slice(0, Math.max(0, scrollLines));
+    const visibleText = beforeLines.join('\n');
+
+    // 找到视口顶部之前最近的标题
+    const headings = [...visibleText.matchAll(/^(#{1,6})\s+(.+)$/gm)];
+    if (!headings.length) return;
+
     const lastH = headings[headings.length - 1];
     const headingText = lastH[2].trim();
 
-    // 在预览区查找对应标题元素
+    // 在预览区查找对应标题
     const previewEls = this.previewEl.querySelectorAll('h1, h2, h3, h4, h5, h6');
+    let found;
     for (const el of previewEls) {
-      if (el.textContent.trim() === headingText) {
-        el.scrollIntoView({ block: 'start', behavior: 'instant' });
-        return;
+      if (el.textContent.trim() === headingText) { found = el; break; }
     }
+    if (found) {
+      found.scrollIntoView({ block: 'start', behavior: 'instant' });
     }
-
-    // fallback
-    const max = ta.scrollHeight - ta.clientHeight;
-    if (max <= 0) return;
-    this.previewPane.scrollTop = (ta.scrollTop / max) * (this.previewPane.scrollHeight - this.previewPane.clientHeight);
   }
 
   _updateWordCount() {