Prices in Event Gallery include tax, so a discount has to reduce the tax as well. The discount is spread across the line items it applies to and the tax share of every portion is tracked, which keeps the VAT correct even when the discounted items use different tax rates. There is nothing to configure for this: the tax rate of a discount is always derived from the items it reduces, so it can never contradict them.

Percentage based payment fees, shipping costs and surcharges are calculated on the undiscounted subtotal. If you want a percentage fee to shrink along with the discount, model it as a promotion instead.