Support setting the layer of a decoration

Fixes #3778

src/common/services/DecorationService.ts

@@ -64,13 +64,13 @@ export class DecorationService extends Disposable implements IDecorationService
     return this._decorations.getKeyIterator(line);
   }
 
-  public *getDecorationsAtCell(x: number, line: number): IterableIterator<IInternalDecoration> {
+  public *getDecorationsAtCell(x: number, line: number, layer?: 'bottom' | 'top'): IterableIterator<IInternalDecoration> {
     let xmin = 0;
     let xmax = 0;
     for (const d of this._decorations.getKeyIterator(line)) {
       xmin = d.options.x ?? 0;
       xmax = xmin + (d.options.width ?? 1);
-      if (x >= xmin && x < xmax) {
+      if (x >= xmin && x < xmax && (!layer || (d.options.layer ?? 'bottom') === layer)) {
         yield d;
       }
     }

src/common/services/Services.ts

@@ -312,7 +312,7 @@ export interface IDecorationService extends IDisposable {
   /** Iterates over the decorations at a line (in no particular order). */
   getDecorationsAtLine(line: number): IterableIterator<IInternalDecoration>;
   /** Iterates over the decorations at a cell (in no particular order). */
-  getDecorationsAtCell(x: number, line: number): IterableIterator<IInternalDecoration>;
+  getDecorationsAtCell(x: number, line: number, layer?: 'bottom' | 'top'): IterableIterator<IInternalDecoration>;
 }
 export interface IInternalDecoration extends IDecoration {
   readonly options: IDecorationOptions;