Items in inventory sorted manually (mouse left click drag) is useful! However this feature is not shown on any PMU controls list I could find. Assuming this feature really is hidden away like this, my suggestion is to include this in whichever guide PMU currently relies on that new players use. If I guess correctly, that would be the PMU Cheat Sheet.
Along with that, we could also mention that, while holding an item, you can keep the item hovered while moving to another page to place an item on another section of your bag.