Mage_CatalogInventory_Model_Stock_Item::suggestQty update

original.php

<?php
public function suggestQty($qty)
{
	// We do not manage stock
	if ($qty <= 0 || !$this->getManageStock()) {
		return $qty;
	}

	$qtyIncrements = (int)$this->getQtyIncrements(); // Currently only integer increments supported
	if ($qtyIncrements < 2) {
		return $qty;
	}

	$minQty = max($this->getMinSaleQty(), $qtyIncrements);
	$divisibleMin = ceil($minQty / $qtyIncrements) * $qtyIncrements;

	$maxQty = min($this->getQty() - $this->getMinQty(), $this->getMaxSaleQty());
	$divisibleMax = floor($maxQty / $qtyIncrements) * $qtyIncrements;

	if ($qty < $minQty || $qty > $maxQty || $divisibleMin > $divisibleMax) {
		// Do not perform rounding for qty that does not satisfy min/max conditions to not confuse customer
		return $qty;
	}

	// Suggest value closest to given qty
	$closestDivisibleLeft = floor($qty / $qtyIncrements) * $qtyIncrements;
	$closestDivisibleRight = $closestDivisibleLeft + $qtyIncrements;
	$acceptableLeft = min(max($divisibleMin, $closestDivisibleLeft), $divisibleMax);
	$acceptableRight = max(min($divisibleMax, $closestDivisibleRight), $divisibleMin);
	return abs($acceptableLeft - $qty) < abs($acceptableRight - $qty) ? $acceptableLeft : $acceptableRight;
}

updated.php

<?php
public function suggestQty($qty)
{
	// Handle only number quantities and only if stock management is enabled
	if (is_numeric($qty) && $this->getManageStock()) {
		// Grab and type-cast the qty increment to a float
		$qtyIncrements = (float)$this->getQtyIncrements();
		
		// If increment is 0 (or less) then increments are disabled, no need to round the qty
		if ($qtyIncrements > 0) {
			// Type-cast the qty to a float
			$qty = (float)$qty;
			
			// Get a min that respects the increment
			$minQty = ceil((float)$this->getMinSaleQty() / $qtyIncrements) * $qtyIncrements;
			
			// Get a max that respects the increment
			$maxQty = floor((float)$this->getMaxSaleQty() / $qtyIncrements) * $qtyIncrements;
			
			// If the qty is out of range of min/max or min/max is invalid then skip the rounding
			if ($minQty < $maxQty && $qty > $minQty && $qty < $maxQty) {
				// Get a qty that respects the increment
				$qty = round($qty / $qtyIncrements) * $qtyIncrements;
			}
		}
	}

	return $qty;
}

This one fully handles fractional increments and doesn't use the crazy decision logic in the old one.