Method compositeLineAt

packages/pi-tui/src/tui.ts:1186–1234

Splice overlay content into a base line at a specific column. Single-pass optimized.

(
		baseLine: string,
		overlayLine: string,
		startCol: number,
		overlayWidth: number,
		totalWidth: number,
	)

	/** Splice overlay content into a base line at a specific column. Single-pass optimized. */
	private compositeLineAt(
		baseLine: string,
		overlayLine: string,
		startCol: number,
		overlayWidth: number,
		totalWidth: number,
	): string {
		if (isImageLine(baseLine)) return baseLine;

		// Single pass through baseLine extracts both before and after segments
		const afterStart = startCol + overlayWidth;
		const base = extractSegments(baseLine, startCol, afterStart, totalWidth - afterStart, true);

		// Extract overlay with width tracking (strict=true to exclude wide chars at boundary)
		const overlay = sliceWithWidth(overlayLine, 0, overlayWidth, true);

		// Pad segments to target widths
		const beforePad = Math.max(0, startCol - base.beforeWidth);
		const overlayPad = Math.max(0, overlayWidth - overlay.width);
		const actualBeforeWidth = Math.max(startCol, base.beforeWidth);
		const actualOverlayWidth = Math.max(overlayWidth, overlay.width);
		const afterTarget = Math.max(0, totalWidth - actualBeforeWidth - actualOverlayWidth);
		const afterPad = Math.max(0, afterTarget - base.afterWidth);

		// Compose result
		const r = TUI.SEGMENT_RESET;
		const result =
			base.before +
			" ".repeat(beforePad) +
			r +
			overlay.text +
			" ".repeat(overlayPad) +
			r +
			base.after +
			" ".repeat(afterPad);

		// CRITICAL: Always verify and truncate to terminal width.
		// This is the final safeguard against width overflow which would crash the TUI.
		// Width tracking can drift from actual visible width due to:
		// - Complex ANSI/OSC sequences (hyperlinks, colors)
		// - Wide characters at segment boundaries
		// - Edge cases in segment extraction
		const resultWidth = visibleWidth(result);
		if (resultWidth <= totalWidth) {
			return result;
		}
		// Truncate with strict=true to ensure we don't exceed totalWidth
		return sliceByColumn(result, 0, totalWidth, true);
	}

	/**
	 * Find and extract cursor position from rendered lines.