Do char atlas warmup via new IdleTaskQueue

addons/xterm-addon-webgl/src/atlas/WebglCharAtlas.ts

@@ -16,6 +16,7 @@ import { tryDrawCustomChar } from 'browser/renderer/CustomGlyphs';
 import { excludeFromContrastRatioDemands, isPowerlineGlyph, isRestrictedPowerlineGlyph } from 'browser/renderer/RendererUtils';
 import { IUnicodeService } from 'common/services/Services';
 import { FourKeyMap } from 'common/MultiKeyMap';
+import { IdleTaskQueue } from 'common/IdleTaskQueue';
 
 // For debugging purposes, it can be useful to set this to a really tiny value,
 // to verify that LRU eviction works.
@@ -124,20 +125,21 @@ export class WebglCharAtlas implements IDisposable {
 
   public warmUp(): void {
     if (!this._didWarmUp) {
-      (typeof requestIdleCallback !== 'function' ? requestIdleCallback : setTimeout)(() => {
-        this._doWarmUp();
-      });
+      this._doWarmUp();
       this._didWarmUp = true;
     }
   }
 
   private _doWarmUp(): void {
-    // Pre-fill with ASCII 33-126
+    // Pre-fill with ASCII 33-126, this is not urgent and done in idle callbacks
+    const queue = new IdleTaskQueue();
     for (let i = 33; i < 126; i++) {
-      if (!this._cacheMap.get(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT)) {
-        const rasterizedGlyph = this._drawToCache(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT);
-        this._cacheMap.set(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT, rasterizedGlyph);
-      }
+      queue.enqueue(() => {
+        if (!this._cacheMap.get(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT)) {
+          const rasterizedGlyph = this._drawToCache(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT);
+          this._cacheMap.set(i, DEFAULT_COLOR, DEFAULT_COLOR, DEFAULT_EXT, rasterizedGlyph);
+        }
+      });
     }
   }
 

src/common/IdleTaskQueue.ts

@@ -0,0 +1,52 @@
+/**
+ * Copyright (c) 2022 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+/**
+ * A queue of that runs tasks over several idle callbacks, trying to maintain the specified
+ * frame rate. The tasks will run in the order they are enqueued, but they will run some time later,
+ * and care should be taken to ensure they're non-urgent and will not introduce race conditions.
+ */
+export class IdleTaskQueue {
+  private _tasks: Function[] = [];
+  private _idleCallback?: number;
+  private _maxTaskDuration: number;
+  private _i = 0;
+
+  /**
+   * @param targetFps The target frame rate.
+   */
+  constructor(targetFps: number = 240) {
+    this._maxTaskDuration = 1000 / targetFps;
+  }
+
+  /**
+   * Adds a task to the queue which will run in a future idle callback.
+   */
+  public enqueue(task: Function): void {
+    this._tasks.push(task);
+    this._start();
+  }
+
+  private _start(): void {
+    if (!this._idleCallback) {
+      this._idleCallback = requestIdleCallback(() => this._process());
+    }
+  }
+
+  private _process(): void {
+    const start = performance.now();
+    this._idleCallback = undefined;
+    while (this._i < this._tasks.length) {
+      this._tasks[this._i++]();
+      if (performance.now() - start > this._maxTaskDuration) {
+        this._start();
+        return;
+      }
+    }
+    // Clear the queue
+    this._i = 0;
+    this._tasks.length = 0;
+  }
+}