Fix E-commerce Site Quantity Selector Issues

PHP, JavaScript, Website Design, CSS, Website Testing, HTML, Debugging, Web Development, Frontend Development, Mobile Development · am looking for a sharp front-end developer/JS expert to debug and fix a quantity selector on my e-commerce product pages. Everything worked flawlessly for months, but after a recent theme update, several frustrating bugs appeared. The custom JavaScript handling this logic was left untouched, but it’s clearly conflicting with the new theme update. I have console error screenshots ready, and a staging site is fully set up so you can work safely without touching the live store. The Bugs We Need Fixed: 1. Desktop Increment Math: Rapidly clicking the "+" button causes the number to jump erratically (e.g., 1 → 3 → 6) instead of going up by 1. 2. Mobile "Dead" First Tap: The first tap on the "+" button does absolutely nothing. Every tap after that works fine. 3. iPhone Safari Scroll Glitch: On iOS Safari specifically, tapping the "+" button sometimes forces the page to scroll instead of changing the quantity. 4. Negative Quantities: The "-" button lacks a zero-floor limit. Users can go below 0, resulting in "-2 items" showing up in the cart. 5. Cart Desync: Occasionally, the cart total fails to update, causing a price mismatch between the page and the actual cart. What I Expect From