blitScrolled method
Blit retained content shifted by deltaLines cells (positive = down).
Implementation
void blitScrolled(Canvas canvas, int deltaLines, double cellHeight, Size size) {
if (!hasContent || deltaLines == 0) return;
canvas.save();
canvas.clipRect(Offset.zero & size);
canvas.translate(0, deltaLines * cellHeight);
blitTo(canvas);
canvas.restore();
}