src/vs/workbench/browser/parts/compositePart.ts/CompositePart/createContentArea
class CompositePart:	override createContentArea(parent: HTMLElement): HTMLElement {
		const contentContainer = append(parent, $('.content'));

		this.progressBar = this._register(new ProgressBar(contentContainer));
		this._register(attachProgressBarStyler(this.progressBar, this.themeService));
		this.progressBar.hide();

		return contentContainer;
	}