1. How to make the scroll to top button appear only after scrolling 50% of the page.
The scroll to top button is generally added by using the displayBeforeBodyClosingTag hook.
1) Adjust the button for a bit, add a stsb_display_none classname to the button. Set it to be the listener in the JS event section, see the first picture below.
2) In the same record, add a container, set it to be at 50% of the page by using the following settings:
- Content Width: Full width
- Width: 1px
- Height: 1px
- Position: Absolute
- Left: 0
- Top: 50%
Set it to be the trigger in the JS event section, see the second picture below.
3) Add this css code to the custom css code field.
body{height:auto;position:relative;} 
2. Add a loader while changing the product combination
Check #10 on this page Product page builder
