feat: improve editor performance (#14429)

#### PR Dependency Tree


* **PR #14429** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* HTML import now splits lines on <br> into separate paragraphs while
preserving inline formatting.

* **Bug Fixes**
* Paste falls back to inserting after the first paragraph when no
explicit target is found.

* **Style**
  * Improved page-mode viewport styling for consistent content layout.

* **Tests**
* Added snapshot tests for <br>-based paragraph splitting; re-enabled an
e2e drag-page test.

* **Chores**
* Deferred/deduplicated font loading, inline text caching,
drag-handle/pointer optimizations, and safer inline render
synchronization.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/blocks/paragraph/src/adapters/html.ts

@@ -37,6 +37,126 @@ const tagsInAncestor = (o: NodeProps<HtmlAST>, tagNames: Array<string>) => {
   return false;
 };
 
+const splitDeltaByNewline = (delta: DeltaInsert[]) => {
+  const lines: DeltaInsert[][] = [[]];
+  const pending = [...delta];
+
+  while (pending.length > 0) {
+    const op = pending.shift();
+    if (!op) continue;
+
+    const insert = op.insert;
+    if (typeof insert !== 'string') {
+      lines[lines.length - 1].push(op);
+      continue;
+    }
+
+    if (!insert.includes('\n')) {
+      if (insert.length === 0) {
+        continue;
+      }
+      lines[lines.length - 1].push(op);
+      continue;
+    }
+
+    const splitIndex = insert.indexOf('\n');
+    const linePart = insert.slice(0, splitIndex);
+    const remainPart = insert.slice(splitIndex + 1);
+    if (linePart.length > 0) {
+      lines[lines.length - 1].push({ ...op, insert: linePart });
+    }
+    lines.push([]);
+    if (remainPart) {
+      pending.unshift({ ...op, insert: remainPart });
+    }
+  }
+
+  return lines;
+};
+
+const hasBlockElementDescendant = (node: HtmlAST): boolean => {
+  if (!HastUtils.isElement(node)) {
+    return false;
+  }
+  return node.children.some(child => {
+    if (!HastUtils.isElement(child)) {
+      return false;
+    }
+    return (
+      (HastUtils.isTagBlock(child.tagName) && child.tagName !== 'br') ||
+      hasBlockElementDescendant(child)
+    );
+  });
+};
+
+const getParagraphDeltas = (
+  node: HtmlAST,
+  delta: DeltaInsert[]
+): DeltaInsert[][] => {
+  if (!HastUtils.isElement(node)) return [delta];
+  if (hasBlockElementDescendant(node)) return [delta];
+
+  const hasBr = !!HastUtils.querySelector(node, 'br');
+  if (!hasBr) return [delta];
+
+  const hasNewline = delta.some(
+    op => typeof op.insert === 'string' && op.insert.includes('\n')
+  );
+  if (!hasNewline) return [delta];
+
+  return splitDeltaByNewline(delta);
+};
+
+const openParagraphBlocks = (
+  deltas: DeltaInsert[][],
+  type: string,
+  // AST walker context from html adapter transform pipeline.
+  walkerContext: any
+) => {
+  for (const delta of deltas) {
+    walkerContext
+      .openNode(
+        {
+          type: 'block',
+          id: nanoid(),
+          flavour: 'affine:paragraph',
+          props: { type, text: { '$blocksuite:internal:text$': true, delta } },
+          children: [],
+        },
+        'children'
+      )
+      .closeNode();
+  }
+};
+
+const MULTI_PARAGRAPH_EMITTED_NODES_CONTEXT_KEY =
+  'affine:paragraph:multi-emitted-nodes';
+
+const markMultiParagraphEmitted = (walkerContext: any, node: HtmlAST) => {
+  const emittedNodes =
+    (walkerContext.getGlobalContext(
+      MULTI_PARAGRAPH_EMITTED_NODES_CONTEXT_KEY
+    ) as WeakSet<object> | undefined) ?? new WeakSet<object>();
+  emittedNodes.add(node as object);
+  walkerContext.setGlobalContext(
+    MULTI_PARAGRAPH_EMITTED_NODES_CONTEXT_KEY,
+    emittedNodes
+  );
+};
+
+const consumeMultiParagraphEmittedMark = (
+  walkerContext: any,
+  node: HtmlAST
+) => {
+  const emittedNodes = walkerContext.getGlobalContext(
+    MULTI_PARAGRAPH_EMITTED_NODES_CONTEXT_KEY
+  ) as WeakSet<object> | undefined;
+  if (!emittedNodes) {
+    return false;
+  }
+  return emittedNodes.delete(node as object);
+};
+
 export const paragraphBlockHtmlAdapterMatcher: BlockHtmlAdapterMatcher = {
   flavour: ParagraphBlockSchema.model.flavour,
   toMatch: o =>
@@ -88,41 +208,37 @@ export const paragraphBlockHtmlAdapterMatcher: BlockHtmlAdapterMatcher = {
             !tagsInAncestor(o, ['p', 'li']) &&
             HastUtils.isParagraphLike(o.node)
           ) {
-            walkerContext
-              .openNode(
-                {
-                  type: 'block',
-                  id: nanoid(),
-                  flavour: 'affine:paragraph',
-                  props: {
-                    type: 'text',
-                    text: {
-                      '$blocksuite:internal:text$': true,
-                      delta: deltaConverter.astToDelta(o.node),
-                    },
-                  },
-                  children: [],
-                },
-                'children'
-              )
-              .closeNode();
+            const delta = deltaConverter.astToDelta(o.node);
+            const deltas = getParagraphDeltas(o.node, delta);
+            openParagraphBlocks(deltas, 'text', walkerContext);
             walkerContext.skipAllChildren();
           }
           break;
         }
         case 'p': {
+          const type = walkerContext.getGlobalContext('hast:blockquote')
+            ? 'quote'
+            : 'text';
+          const delta = deltaConverter.astToDelta(o.node);
+          const deltas = getParagraphDeltas(o.node, delta);
+
+          if (deltas.length > 1) {
+            openParagraphBlocks(deltas, type, walkerContext);
+            markMultiParagraphEmitted(walkerContext, o.node);
+            walkerContext.skipAllChildren();
+            break;
+          }
+
           walkerContext.openNode(
             {
               type: 'block',
               id: nanoid(),
               flavour: 'affine:paragraph',
               props: {
-                type: walkerContext.getGlobalContext('hast:blockquote')
-                  ? 'quote'
-                  : 'text',
+                type,
                 text: {
                   '$blocksuite:internal:text$': true,
-                  delta: deltaConverter.astToDelta(o.node),
+                  delta,
                 },
               },
               children: [],
@@ -192,6 +308,9 @@ export const paragraphBlockHtmlAdapterMatcher: BlockHtmlAdapterMatcher = {
           break;
         }
         case 'p': {
+          if (consumeMultiParagraphEmittedMark(walkerContext, o.node)) {
+            break;
+          }
           if (
             o.next?.type === 'element' &&
             o.next.tagName === 'div' &&