Avoid garbage collection in webgl decoration hot code

Part of #4079

src/common/services/DecorationService.ts

@@ -11,6 +11,12 @@ import { SortedList } from 'common/SortedList';
 import { IColor } from 'common/Types';
 import { IDecorationOptions, IDecoration, IMarker, IEvent } from 'xterm';
 
+/** Work variables to avoid garbage collection. */
+const w = {
+  xmin: 0,
+  xmax: 0
+};
+
 export class DecorationService extends Disposable implements IDecorationService {
   public serviceBrand: any;
 
@@ -72,6 +78,16 @@ export class DecorationService extends Disposable implements IDecorationService
     }
   }
 
+  public forEachDecorationAtCell(x: number, line: number, layer: 'bottom' | 'top' | undefined, callback: (decoration: IInternalDecoration) => void): void {
+    this._decorations.forEachByKey(line, d => {
+      w.xmin = d.options.x ?? 0;
+      w.xmax = w.xmin + (d.options.width ?? 1);
+      if (x >= w.xmin && x < w.xmax && (!layer || (d.options.layer ?? 'bottom') === layer)) {
+        callback(d);
+      }
+    });
+  }
+
   public dispose(): void {
     for (const d of this._decorations.values()) {
       this._onDecorationRemoved.fire(d);

src/common/services/Services.ts

@@ -308,6 +308,11 @@ export interface IDecorationService extends IDisposable {
   getDecorationsAtLine(line: number): IterableIterator<IInternalDecoration>;
   /** Iterates over the decorations at a cell (in no particular order). */
   getDecorationsAtCell(x: number, line: number, layer?: 'bottom' | 'top'): IterableIterator<IInternalDecoration>;
+  /**
+   * Trigger a callback over the decoration at a cell (in no particular order). This is a high
+   * performance, but less ergonomic, version of {@link getDecorationsAtCell}.
+   */
+  forEachDecorationAtCell(x: number, line: number, layer: 'bottom' | 'top' | undefined, callback: (decoration: IInternalDecoration) => void): void;
 }
 export interface IInternalDecoration extends IDecoration {
   readonly options: IDecorationOptions;