feat(typesetting): 分页算法增加orphan control - 标题不再孤悬页面底部

- paginateContent正常块判断增加标题orphan control
- 标题(h2/h3/h4)后检查下一个块能否放入当前页
- 若放不下则回退标题到下一页开头
- 超大块处理增加标题回退逻辑
- 超大表格拆分时前面的h2标题和表格第一部分同页

public/static/js/phone-image.js

@@ -615,6 +615,15 @@ var PhoneImageEngine = (function () {
 
             // 超大块处理：单个块超过整页高度时需要拆分
             if (block.estimatedHeight > contentAreaHeight) {
+                // 检查当前页最后一个块是否为标题，若是则回退使其与超大块第一部分同页
+                var lastBlockInPage = currentPageBlocks.length > 0 ? currentPageBlocks[currentPageBlocks.length - 1] : null;
+                var lastBlockIsHeading = lastBlockInPage && (lastBlockInPage.type === 'h2' || lastBlockInPage.type === 'h3' || lastBlockInPage.type === 'h4');
+
+                if (lastBlockIsHeading) {
+                    currentPageBlocks.pop();
+                    currentHeight -= lastBlockInPage.estimatedHeight;
+                }
+
                 // 先把当前页已有的内容推出去
                 if (currentPageBlocks.length > 0) {
                     contentPages.push(generateContentPage(
@@ -625,6 +634,12 @@ var PhoneImageEngine = (function () {
                     pageNumber++;
                 }
 
+                // 超大块拆分后，若有回退的标题，加到新页开头
+                if (lastBlockIsHeading) {
+                    currentPageBlocks = [lastBlockInPage];
+                    currentHeight = lastBlockInPage.estimatedHeight;
+                }
+
                 // 尝试拆分超大块
                 var splitBlocks = splitOversizedBlock(block, contentAreaHeight);
                 for (var s = 0; s < splitBlocks.length; s++) {
@@ -644,6 +659,9 @@ var PhoneImageEngine = (function () {
             }
 
             // 正常块：判断是否需要换页
+            var isHeading = block.type === 'h2' || block.type === 'h3' || block.type === 'h4';
+            var nextBlock = (i + 1 < blocks.length) ? blocks[i + 1] : null;
+
             if (currentHeight + block.estimatedHeight > contentAreaHeight && currentPageBlocks.length > 0) {
                 contentPages.push(generateContentPage(
                     currentPageBlocks, pageNumber, sizeConfig, false
@@ -655,6 +673,22 @@ var PhoneImageEngine = (function () {
 
             currentPageBlocks.push(block);
             currentHeight += block.estimatedHeight;
+
+            // Orphan control: 标题后至少跟一个内容块，若下一个块放不下则标题移到下一页
+            if (isHeading && nextBlock) {
+                if (currentHeight + nextBlock.estimatedHeight > contentAreaHeight) {
+                    currentPageBlocks.pop();
+                    currentHeight -= block.estimatedHeight;
+                    if (currentPageBlocks.length > 0) {
+                        contentPages.push(generateContentPage(
+                            currentPageBlocks, pageNumber, sizeConfig, false
+                        ));
+                        pageNumber++;
+                    }
+                    currentPageBlocks = [block];
+                    currentHeight = block.estimatedHeight;
+                }
+            }
         }
 
         // 最后一页