Fix border collapsing when borderStyle changes

src/reconciler.ts

@@ -95,6 +95,30 @@ type UpdatePayload = {
 	style: Styles | undefined;
 };
 
+const finalizeChangedStyle = (
+	changedStyle: Styles | undefined,
+	currentStyle: Styles | undefined,
+): Styles | undefined => {
+	if (!changedStyle) {
+		return;
+	}
+
+	if (!currentStyle) {
+		return changedStyle;
+	}
+
+	const style = {...changedStyle};
+
+	if ('borderStyle' in style) {
+		style.borderTop = currentStyle.borderTop;
+		style.borderRight = currentStyle.borderRight;
+		style.borderBottom = currentStyle.borderBottom;
+		style.borderLeft = currentStyle.borderLeft;
+	}
+
+	return style;
+};
+
 export default createReconciler<
 	ElementNames,
 	Props,
@@ -254,15 +278,20 @@ export default createReconciler<
 
 		const props = diff(oldProps, newProps);
 
-		const style = diff(
+		const changedStyle = diff(
 			oldProps['style'] as Styles,
 			newProps['style'] as Styles,
 		);
 
-		if (!props && !style) {
+		if (!props && !changedStyle) {
 			return null;
 		}
 
+		const style = finalizeChangedStyle(
+			changedStyle as Styles,
+			newProps['style'] as Styles,
+		);
+
 		return {props, style};
 	},
 	commitUpdate(node, {props, style}) {

test/borders.tsx

@@ -4,7 +4,7 @@ import boxen, {type Options} from 'boxen';
 import indentString from 'indent-string';
 import delay from 'delay';
 import widestLine from 'widest-line';
-import cliBoxes from 'cli-boxes';
+import cliBoxes, {type Boxes} from 'cli-boxes';
 import chalk from 'chalk';
 import {render, Box, Text} from '../src/index.js';
 import {renderToString} from './helpers/render-to-string.js';
@@ -613,6 +613,35 @@ test('hide all borders', t => {
 	t.is(output, ['Above', 'Content', 'Below'].join('\n'));
 });
 
+test('keep borders hidden on `borderStyle` change', async t => {
+	const stdout = createStdout();
+
+	function Test() {
+		const [state, setState] = useState<keyof Boxes>('single');
+
+		useEffect(() => {
+			setState('double');
+		}, []);
+
+		return (
+			<Box
+				borderStyle={state}
+				borderTop={false}
+				borderBottom={false}
+				borderLeft={false}
+				borderRight={false}
+			>
+				<Text>Content</Text>
+			</Box>
+		);
+	}
+
+	render(<Test />, {stdout, debug: true});
+	t.is((stdout.write as any).firstCall.args[0], 'Content');
+	await delay(100);
+	t.is((stdout.write as any).lastCall.args[0], 'Content');
+});
+
 test('change color of top border', t => {
 	const output = renderToString(
 		<Box flexDirection="column" alignItems="flex-start">