I found a way to do this in 3 super quick steps- so sharing it below:
- Press CTRL+A to select all items (Ensure the completed items are hidden - this should be default) and then Press CTRL+D to mark them as completed.
- Now all items should be hidden since they are marked as complete. Now click 'Show marked items' to show them all.
- CTRL+A to select all and CTRL+D to mark them incomplete.
This has really helped me with my grocery lists since I would need to reset it before the next purchase.