Steps to reproduce:
- Product A with a price-included tax, product B with a negative price and another tax (or none)
- Promotion with a fixed amount discount on specific products (or on the order), both products eligible
- Add both products to a PoS order

Issue:
The discount is split per tax group with a common factor, so the negative line gets a positive reward line, which is expected. But the reward lines add up to less than the discount and the customer pays too much.

Cause:
Since 7280f3597ece each tax group is capped with
`Math.min(get_total_with_tax(), amount)`. A single tax group is compared with the total of the whole order: here the total includes the negative line, so the positive group is lowered while the negative one is kept as is.

Fix:
Apply the cap proportionally: every tax group is scaled by the ratio between the order total and the discountable amount when the total is lower. The scenario of 7280f3597ece gives the same result, and the reward lines add up to the discount whatever the number of tax groups.

opw-6567973

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Forward-Port-Of: odoo/odoo#288853

Before this commit, the test "discard stale mention when replacing it with a longer partner mention" could fail with:

    Failed to find 1 of ".o-mail-Composer-suggestion strong" with text
    "John" (Timeout of 10 seconds). Found 0 instead.

This happens because the mocked get_mention_suggestions_from_channel finds the partners through their users, while python also returns the matching channel partners that have no user. John has no user, so the suggestion list only finds him among the members loaded by /discuss/channel/members. The suggestions are filtered when the term changes and when the mention answer arrives, so John never shows when those members arrive after the mention answer.

This comes from "[FIX] mail: should be able to mention non-members in accessible channels", which replaced the mock filter on the channel partners by a search on the users.

This commit fixes the issue by also returning the channel partners without a user that match the term, as python does.

https://runbot.odoo.com/odoo/error/947558