src/vs/workbench/browser/parts/editor/editorDropTarget.ts/DropOverlay/getOverlayOffsetHeight
class DropOverlay:	private getOverlayOffsetHeight(): number {

		// With tabs and opened editors: use the area below tabs as drop target
		if (!this.groupView.isEmpty && this.accessor.partOptions.showTabs) {
			return this.groupView.titleHeight.offset;
		}

		// Without tabs or empty group: use entire editor area as drop target
		return 0;
	}