src/vs/base/browser/ui/splitview/paneview.ts/PaneView/getPaneSize
class PaneView:	getPaneSize(pane: Pane): number {
		const index = this.paneItems.findIndex(item => item.pane === pane);

		if (index === -1) {
			return -1;
		}

		return this.splitview.getViewSize(index);
	}