src/vs/editor/contrib/folding/browser/foldingDecorations.ts/FoldingDecorationProvider/getDecorationOption
class FoldingDecorationProvider:	getDecorationOption(isCollapsed: boolean, isHidden: boolean, isManual: boolean): IModelDecorationOptions {
		if (isHidden) { // is inside another collapsed region
			return FoldingDecorationProvider.HIDDEN_RANGE_DECORATION;
		}
		if (this.showFoldingControls === 'never') {
			if (isCollapsed) {
				return this.showFoldingHighlights ? FoldingDecorationProvider.NO_CONTROLS_COLLAPSED_HIGHLIGHTED_RANGE_DECORATION : FoldingDecorationProvider.NO_CONTROLS_COLLAPSED_RANGE_DECORATION;
			}
			return FoldingDecorationProvider.NO_CONTROLS_EXPANDED_RANGE_DECORATION;
		}
		if (isCollapsed) {
			return isManual ?
				(this.showFoldingHighlights ? FoldingDecorationProvider.MANUALLY_COLLAPSED_HIGHLIGHTED_VISUAL_DECORATION : FoldingDecorationProvider.MANUALLY_COLLAPSED_VISUAL_DECORATION)
				: (this.showFoldingHighlights ? FoldingDecorationProvider.COLLAPSED_HIGHLIGHTED_VISUAL_DECORATION : FoldingDecorationProvider.COLLAPSED_VISUAL_DECORATION);
		} else if (this.showFoldingControls === 'mouseover') {
			return isManual ? FoldingDecorationProvider.MANUALLY_EXPANDED_AUTO_HIDE_VISUAL_DECORATION : FoldingDecorationProvider.EXPANDED_AUTO_HIDE_VISUAL_DECORATION;
		} else {
			return isManual ? FoldingDecorationProvider.MANUALLY_EXPANDED_VISUAL_DECORATION : FoldingDecorationProvider.EXPANDED_VISUAL_DECORATION;
		}
	}