h * lines.length + pad * 2;
                const bx = 10, by = 10, rr = 6;

                // 半透明白底圆角
                ctx.fillStyle = 'rgba(255,255,255,0.5)';
                ctx.beginPath();
                ctx.moveTo(bx + rr, by);
                ctx.lineTo(bx + boxW - rr, by);
                ctx.quadraticCurveTo(bx + boxW, by, bx + boxW, by + rr);
                ctx.lineTo(bx + boxW, by + boxH - rr);
                ctx.quadraticCurveTo(bx + boxW, by + boxH, bx + boxW - rr, by + boxH);
