src/vs/workbench/common/editor/editorGroupModel.ts/EditorGroupModel/doUnpin
class EditorGroupModel:	private doUnpin(editor: EditorInput, editorIndex: number): void {
		if (!this.isPinned(editor)) {
			return; // can only unpin a pinned editor
		}

		// Set new
		const oldPreview = this.preview;
		this.preview = editor;

		// Event
		const event: IGroupEditorChangeEvent = {
			kind: GroupModelChangeKind.EDITOR_PIN,
			editor,
			editorIndex
		};
		this._onDidModelChange.fire(event);

		// Close old preview editor if any
		if (oldPreview) {
			this.closeEditor(oldPreview, EditorCloseContext.UNPIN);
		}
	}