src/vs/workbench/browser/parts/editor/editorPart.ts/EditorPart/doUpdateMostRecentActive
class EditorPart:	private doUpdateMostRecentActive(group: IEditorGroupView, makeMostRecentlyActive?: boolean): void {
		const index = this.mostRecentActiveGroups.indexOf(group.id);

		// Remove from MRU list
		if (index !== -1) {
			this.mostRecentActiveGroups.splice(index, 1);
		}

		// Add to front as needed
		if (makeMostRecentlyActive) {
			this.mostRecentActiveGroups.unshift(group.id);
		}
	}