fix: single-side borders vanish on a one-row box

[decleezy]

What you expected

A Box with borderLeft draws its border regardless of how tall the box is. A left-only border is a single vertical bar, so height 1 is perfectly drawable.

What happened

The border is silently dropped when the box's content is exactly one row tall. Two rows of content, and it appears.

1 Text, 1 line      NO BORDER   |left       one|
1 Text, 2 lines     border      |left      │one|

Minimal reproduction

import React from "react";
import { Box, Text } from "@orchetron/storm";
import { renderForTest } from "@orchetron/storm/testing";

const BORDER = {
  borderStyle: "single" as const,
  borderLeft: true,
  borderTop: false,
  borderBottom: false,
  borderRight: false,
};

function Split({ lines }: { readonly lines: readonly string[] }) {
  return (
    <Box flexDirection="column" width={40} height={6}>
      <Box flexDirection="row" width={40}>
        <Box width={10}>
          <Text>left</Text>
        </Box>
        <Box width={12} flexDirection="column" {...BORDER}>
          {lines.map((line) => (
            <Text key={line}>{line}</Text>
          ))}
        </Box>
      </Box>
    </Box>
  );
}

for (const lines of [["one"], ["one", "two"]]) {
  const result = renderForTest(<Split lines={lines} />, { width: 40, height: 6 });
  const row = (result.lines[0] ?? "").replace(/\s+$/, "");
  console.log(`${lines.length} row(s): ${row.includes("│") ? "border" : "NO BORDER"}  |${row}|`);
  result.unmount();
}

Cause

paintBorder in src/reconciler/renderer.ts opens with:

if (style === "none" || width < 2 || height < 2) return;

The 2x2 minimum is correct for a full border, since one row cannot hold both a top and a bottom edge plus content. It is not correct for a single-side border.

The change

The guard is narrowed rather than removed. Two cells are needed only on an axis that draws both of its sides:

const minWidth = sides.left && sides.right ? 2 : 1;
const minHeight = sides.top && sides.bottom ? 2 : 1;

A box one column wide still refuses a left-and-right border, which the third test covers.

The rest of paintBorder already handles height 1 correctly: the corner branch writes the single │, and the for (let i = 1; i < height - 1; i++) edge loop does not run.

Testing

Three regression tests in src/__tests__/component-boundary-layout.test.ts (the one-row case, the two-row case, and the narrowed guard still refusing what cannot fit).

• npx tsc --noEmit — 0 errors
• npx vitest run — 547 passed, across three consecutive runs

Context

Found while evaluating Storm as a renderer for a terminal UI with side-by-side panes, where a column's divider is a left-only border. A one-row column loses its divider, which reads as a rendering fault rather than a layout choice. Thanks for the library, the cell diffing is a genuinely different asymptote from a full repaint.