fix: 修复 Markdown 代码块展开动画高度计算异常的问题

src/lib/components/MarkdownRenderer.svelte

@@ -400,14 +400,35 @@
 					codeBlock.classList.remove('is-open');
 					codeBlock.classList.add('is-closed');
 				} else {
+					// 暂时关闭 transition 以获取真实的 scrollHeight (包括 padding)
+					const originalTransition = codeEl.style.transition;
+					codeEl.style.transition = 'none';
 					codeBlock.classList.remove('is-closed');
 					codeBlock.classList.add('is-open');
-					codeEl.style.maxHeight = codeEl.scrollHeight + 'px';
+					codeEl.style.maxHeight = 'none';
+					
+					const targetHeight = codeEl.scrollHeight;
+					
+					// 恢复关闭状态，准备开始动画
+					codeBlock.classList.remove('is-open');
+					codeBlock.classList.add('is-closed');
+					codeEl.style.maxHeight = '0px';
+					void codeEl.offsetHeight; // 强制 reflow
+					
+					// 恢复 transition 并开始动画
+					codeEl.style.transition = originalTransition;
+					codeBlock.classList.remove('is-closed');
+					codeBlock.classList.add('is-open');
+					codeEl.style.maxHeight = targetHeight + 'px';
+					
 					const handler = (e: Event) => {
-						if (e.target === codeEl && codeBlock.classList.contains('is-open')) {
+						const transEvent = e as TransitionEvent;
+						if (e.target === codeEl && transEvent.propertyName === 'max-height') {
+							if (codeBlock.classList.contains('is-open')) {
 								codeEl.style.maxHeight = 'none';
 							}
 							codeEl.removeEventListener('transitionend', handler);
+						}
 					};
 					codeEl.addEventListener('transitionend', handler);
 				}