perf(editor): fallback to placeholder for canvas text (#12033)

### TL;DR

For canvas elements, this PR adds placeholders during zooming operations to improve performance.

![image.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/lEGcysB4lFTEbCwZ8jMv/8c8daea8-1eb4-419b-a4f4-2a8847f40b7b.png)

### What changed?

- Implemented placeholder rendering during zooming operations in the canvas renderer, but not only DOM.
- Added a `forceFullRender` property to the `GfxCompatibleInterface` to allow elements to opt out of placeholder rendering
- Set `forceFullRender = true` for connectors to ensure they always render properly, even during zooming
- Connected the turbo renderer to the viewport's zooming state to automatically switch between full and placeholder rendering

### Why make this change?

Rendering complex elements during zooming operations can cause performance issues and make the UI feel sluggish. Rendering connector label also leads to high cost DOM `set font` delays.

![image.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/lEGcysB4lFTEbCwZ8jMv/961fb847-24b4-4a7f-b9dc-21b0a5edaaa1.png)

The turbo renderer improves performance by displaying simple placeholders for elements during zooming, while still rendering critical elements like connectors fully. This creates a smoother user experience while maintaining essential visual information.

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

## Summary by CodeRabbit

- **New Features**
  - Introduced a feature-flag-controlled "turbo" rendering mode that displays placeholder graphics during zooming for improved performance.
  - Added the ability to override placeholder rendering for specific elements, ensuring full rendering when required.

- **Bug Fixes**
  - Enhanced rendering logic to ensure connectors always render fully, even during zoom operations.

- **Documentation**
  - Updated API documentation to reflect new properties related to rendering behavior.

- **Tests**
  - Improved tests to verify correct rendering behavior for connectors.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/blocks/surface/src/renderer/canvas-renderer.ts

@@ -1,10 +1,12 @@
 import { type Color, ColorScheme } from '@blocksuite/affine-model';
+import { FeatureFlagService } from '@blocksuite/affine-shared/services';
 import { requestConnectedFrame } from '@blocksuite/affine-shared/utils';
 import { DisposableGroup } from '@blocksuite/global/disposable';
 import type { IBound } from '@blocksuite/global/gfx';
 import { getBoundWithRotation, intersects } from '@blocksuite/global/gfx';
 import type { BlockStdScope } from '@blocksuite/std';
 import type {
+  GfxCompatibleInterface,
   GridManager,
   LayerManager,
   SurfaceBlockModel,
@@ -43,6 +45,8 @@ export class CanvasRenderer {
 
   private readonly _disposables = new DisposableGroup();
 
+  private readonly _turboEnabled: () => boolean;
+
   private readonly _overlays = new Set<Overlay>();
 
   private _refreshRafId: number | null = null;
@@ -67,6 +71,8 @@ export class CanvasRenderer {
     removed: HTMLCanvasElement[];
   }>();
 
+  usePlaceholder = false;
+
   viewport: Viewport;
 
   get stackingCanvas() {
@@ -83,6 +89,12 @@ export class CanvasRenderer {
     this.layerManager = options.layerManager;
     this.grid = options.gridManager;
     this.provider = options.provider ?? {};
+
+    this._turboEnabled = () => {
+      const featureFlagService = options.std.get(FeatureFlagService);
+      return featureFlagService.getFlag('enable_turbo_renderer');
+    };
+
     this._initViewport();
 
     options.enableStackingCanvas = options.enableStackingCanvas ?? false;
@@ -213,6 +225,19 @@ export class CanvasRenderer {
         }, this._container);
       })
     );
+
+    this._disposables.add(
+      this.viewport.zooming$.subscribe(isZooming => {
+        const shouldRenderPlaceholders = this._turboEnabled() && isZooming;
+
+        if (this.usePlaceholder !== shouldRenderPlaceholders) {
+          this.usePlaceholder = shouldRenderPlaceholders;
+          this.refresh();
+        }
+      })
+    );
+
+    this.usePlaceholder = false;
   }
 
   private _render() {
@@ -279,23 +304,30 @@ export class CanvasRenderer {
     for (const element of elements) {
       const display = (element.display ?? true) && !element.hidden;
       if (display && intersects(getBoundWithRotation(element), bound)) {
-        const renderFn = this.std.getOptional<ElementRenderer>(
-          ElementRendererIdentifier(element.type)
-        );
+        if (
+          this.usePlaceholder &&
+          !(element as GfxCompatibleInterface).forceFullRender
+        ) {
+          ctx.save();
+          ctx.fillStyle = 'rgba(200, 200, 200, 0.5)';
+          const drawX = element.x - bound.x;
+          const drawY = element.y - bound.y;
+          ctx.fillRect(drawX, drawY, element.w, element.h);
+          ctx.restore();
+        } else {
+          ctx.save();
+          const renderFn = this.std.getOptional<ElementRenderer>(
+            ElementRendererIdentifier(element.type)
+          );
 
-        if (!renderFn) {
-          console.warn(`Cannot find renderer for ${element.type}`);
-          continue;
+          if (!renderFn) continue;
+
+          ctx.globalAlpha = element.opacity ?? 1;
+          const dx = element.x - bound.x;
+          const dy = element.y - bound.y;
+          renderFn(element, ctx, matrix.translate(dx, dy), this, rc, bound);
+          ctx.restore();
         }
-
-        ctx.save();
-
-        ctx.globalAlpha = element.opacity ?? 1;
-        const dx = element.x - bound.x;
-        const dy = element.y - bound.y;
-
-        renderFn(element, ctx, matrix.translate(dx, dy), this, rc, bound);
-        ctx.restore();
       }
     }
 

blocksuite/affine/gfx/connector/src/element-renderer/index.ts

@@ -249,13 +249,20 @@ function renderLabel(
   const [, , w, h] = labelXYWH!;
   const cx = w / 2;
   const cy = h / 2;
+
+  ctx.setTransform(matrix);
+
+  if (renderer.usePlaceholder) {
+    ctx.fillStyle = 'rgba(200, 200, 200, 0.5)';
+    ctx.fillRect(0, 0, w, h);
+    return; // Skip actual label rendering
+  }
+
   const deltas = wrapTextDeltas(text!, font, w);
   const lines = deltaInsertsToChunks(deltas);
   const lineHeight = getLineHeight(fontFamily, fontSize, fontWeight);
   const textHeight = (lines.length - 1) * lineHeight * 0.5;
 
-  ctx.setTransform(matrix);
-
   ctx.font = font;
   ctx.textAlign = textAlign;
   ctx.textBaseline = 'middle';

blocksuite/affine/model/src/elements/connector/connector.ts

@@ -106,6 +106,11 @@ export type ConnectorElementProps = BaseElementProps & {
 export class ConnectorElementModel extends GfxPrimitiveElementModel<ConnectorElementProps> {
   updatingPath = false;
 
+  /**
+   * Connectors should always render, even during zoom.
+   */
+  forceFullRender = true;
+
   override get connectable() {
     return false as const;
   }

blocksuite/affine/shared/src/services/feature-flag-service.ts

@@ -19,6 +19,7 @@ export interface BlockSuiteFlags {
   enable_callout: boolean;
   enable_edgeless_scribbled_style: boolean;
   enable_embed_doc_with_alias: boolean;
+  enable_turbo_renderer: boolean;
 }
 
 export class FeatureFlagService extends StoreExtension {
@@ -42,6 +43,7 @@ export class FeatureFlagService extends StoreExtension {
     enable_callout: false,
     enable_edgeless_scribbled_style: false,
     enable_embed_doc_with_alias: false,
+    enable_turbo_renderer: false,
   });
 
   setFlag(key: keyof BlockSuiteFlags, value: boolean) {