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 -->

packages/frontend/core/src/desktop/pages/workspace/detail-page/detail-page.css.ts

@@ -1,6 +1,6 @@
 import { cssVar } from '@toeverything/theme';
 import { cssVarV2 } from '@toeverything/theme/v2';
-import { style } from '@vanilla-extract/css';
+import { globalStyle, style } from '@vanilla-extract/css';
 
 export const mainContainer = style({
   containerType: 'inline-size',
@@ -47,6 +47,12 @@ export const affineDocViewport = style({
   },
 });
 
+export const pageModeViewportContentBox = style({});
+globalStyle(`${pageModeViewportContentBox} >:first-child`, {
+  display: 'table !important',
+  minWidth: '100%',
+});
+
 export const scrollbar = style({
   marginRight: '4px',
 });

packages/frontend/core/src/desktop/pages/workspace/detail-page/detail-page.tsx

@@ -347,7 +347,8 @@ const DetailPageImpl = memo(function DetailPageImpl() {
                 className={clsx(
                   'affine-page-viewport',
                   styles.affineDocViewport,
-                  styles.editorContainer
+                  styles.editorContainer,
+                  { [styles.pageModeViewportContentBox]: mode === 'page' }
                 )}
               >
                 <PageDetailEditor onLoad={onLoad} readonly={readonly} />