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/root/src/clipboard/page-clipboard.ts

@@ -86,6 +86,7 @@ export class PageClipboard extends ReadOnlyClipboard {
 
     if (this.std.store.readonly) return;
     this.std.store.captureSync();
+    let hasPasteTarget = false;
     this.std.command
       .chain()
       .try<{}>(cmd => [
@@ -144,18 +145,39 @@ export class PageClipboard extends ReadOnlyClipboard {
         if (!ctx.parentBlock) {
           return;
         }
+        hasPasteTarget = true;
         this.std.clipboard
           .paste(
             e,
             this.std.store,
             ctx.parentBlock.model.id,
-            ctx.blockIndex ? ctx.blockIndex + 1 : 1
+            ctx.blockIndex !== undefined ? ctx.blockIndex + 1 : 1
           )
           .catch(console.error);
 
         return next();
       })
       .run();
+
+    if (hasPasteTarget) return;
+
+    // If no valid selection target exists (for example, stale block selection
+    // right after cut), create/focus the default paragraph and paste after it.
+    const firstParagraphId = document
+      .querySelector('affine-page-root')
+      ?.focusFirstParagraph?.()?.id;
+    const parentModel = firstParagraphId
+      ? this.std.store.getParent(firstParagraphId)
+      : null;
+    const paragraphIndex =
+      firstParagraphId && parentModel
+        ? parentModel.children.findIndex(child => child.id === firstParagraphId)
+        : -1;
+    const insertIndex = paragraphIndex >= 0 ? paragraphIndex + 1 : undefined;
+
+    this.std.clipboard
+      .paste(e, this.std.store, parentModel?.id, insertIndex)
+      .catch(console.error);
   };
 
   override mounted() {