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/framework/std/src/inline/utils/text.ts

@@ -8,6 +8,92 @@ export function calculateTextLength(text: Text): number {
   }
 }
 
+type InlineRootTextCache = {
+  dirty: boolean;
+  observer: MutationObserver | null;
+  textNodes: Text[];
+  textNodeIndexMap: WeakMap<Text, number>;
+  prefixLengths: number[];
+  lineIndexMap: WeakMap<Element, number>;
+};
+
+const inlineRootTextCaches = new WeakMap<HTMLElement, InlineRootTextCache>();
+
+const buildInlineRootTextCache = (
+  rootElement: HTMLElement,
+  cache: InlineRootTextCache
+) => {
+  const textSpanElements = Array.from(
+    rootElement.querySelectorAll('[data-v-text="true"]')
+  );
+  const textNodes: Text[] = [];
+  const textNodeIndexMap = new WeakMap<Text, number>();
+  const prefixLengths: number[] = [];
+  let prefixLength = 0;
+
+  for (const textSpanElement of textSpanElements) {
+    const textNode = Array.from(textSpanElement.childNodes).find(
+      (node): node is Text => node instanceof Text
+    );
+    if (!textNode) continue;
+    prefixLengths.push(prefixLength);
+    textNodeIndexMap.set(textNode, textNodes.length);
+    textNodes.push(textNode);
+    prefixLength += calculateTextLength(textNode);
+  }
+
+  const lineIndexMap = new WeakMap<Element, number>();
+  const lineElements = Array.from(rootElement.querySelectorAll('v-line'));
+  for (const [index, line] of lineElements.entries()) {
+    lineIndexMap.set(line, index);
+  }
+
+  cache.textNodes = textNodes;
+  cache.textNodeIndexMap = textNodeIndexMap;
+  cache.prefixLengths = prefixLengths;
+  cache.lineIndexMap = lineIndexMap;
+  cache.dirty = false;
+};
+
+export function invalidateInlineRootTextCache(rootElement: HTMLElement) {
+  const cache = inlineRootTextCaches.get(rootElement);
+  if (cache) {
+    cache.dirty = true;
+  }
+}
+
+export function getInlineRootTextCache(rootElement: HTMLElement) {
+  let cache = inlineRootTextCaches.get(rootElement);
+  if (!cache) {
+    cache = {
+      dirty: true,
+      observer: null,
+      textNodes: [],
+      textNodeIndexMap: new WeakMap(),
+      prefixLengths: [],
+      lineIndexMap: new WeakMap(),
+    };
+    inlineRootTextCaches.set(rootElement, cache);
+  }
+
+  if (!cache.observer && typeof MutationObserver !== 'undefined') {
+    cache.observer = new MutationObserver(() => {
+      cache!.dirty = true;
+    });
+    cache.observer.observe(rootElement, {
+      subtree: true,
+      childList: true,
+      characterData: true,
+    });
+  }
+
+  if (cache.dirty) {
+    buildInlineRootTextCache(rootElement, cache);
+  }
+
+  return cache;
+}
+
 export function getTextNodesFromElement(element: Element): Text[] {
   const textSpanElements = Array.from(
     element.querySelectorAll('[data-v-text="true"]')