src/vs/workbench/contrib/debug/browser/repl.ts/Repl/updateInputDecoration
class Repl:	private updateInputDecoration(): void {
		if (!this.replInput) {
			return;
		}

		const decorations: IDecorationOptions[] = [];
		if (this.isReadonly && this.replInput.hasTextFocus() && !this.replInput.getValue()) {
			const transparentForeground = resolveColorValue(editorForeground, this.themeService.getColorTheme())?.transparent(0.4);
			decorations.push({
				range: {
					startLineNumber: 0,
					endLineNumber: 0,
					startColumn: 0,
					endColumn: 1
				},
				renderOptions: {
					after: {
						contentText: localize('startDebugFirst', "Please start a debug session to evaluate expressions"),
						color: transparentForeground ? transparentForeground.toString() : undefined
					}
				}
			});
		}

		this.replInput.setDecorations('repl-decoration', DECORATION_KEY, decorations);
	}