A user reported to me that they are unable to create a PSBT with a legacy watchonly wallet when they use the GUI. They get an error "The amount exceeds your balance".

I think the issue is that the GUI will retrieve the wallet's balance via `AvailableCoins` before calling `CreateTransaction`. This call to `AvailableCoins` is resulting in fewer coins being returned with recent changes to how `AvailableCoins` works, particularly with additional filtering of spendable coins and preset coins.

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Note that this does not effect descriptor wallets. A workaround for this issue is to switch to using descriptor wallets. An existing legacy wallet can be turned into a descriptor wallet using the `migratewallet` RPC. However many watchonly legacy wallets are likely the result of importing descriptors via `importmulti`. For such wallets, I recommend creating a new descriptor wallet and re-importing the descriptors.
coin_selection_params.m_cost_of_change = coin_selection_params.m_discard_feerate.GetFee(coin_selection_params.change_spend_size) + coin_selection_params.m_change_fee;
    coin_selection_params.m_min_change_target = GenerateChangeTarget(std::floor(recipients_sum / vecSend.size()), coin_selection_params.m_change_fee, rng_fast);
    
    
    
    const auto dust = GetDustThreshold(change_prototype_txout, coin_selection_params.m_discard_feerate);
    const auto change_spend_fee = coin_selection_params.m_discard_feerate.GetFee(coin_selection_params.change_spend_size);
    coin_selection_params.min_viable_change = std::max(change_spend_fee + 1, dust);
    
    coin_selection_params.tx_noinputs_size = 10 + GetSizeOfCompactSize(vecSend.size()); 
    
    for (const auto& recipient : vecSend)
    {
        CTxOut txout(recipient.nAmount, recipient.scriptPubKey);
        
        coin_selection_params.tx_noinputs_size += ::GetSerializeSize(txout, PROTOCOL_VERSION);
        if (IsDust(txout, wallet.chain().relayDustFee())) {
            return util::Error{_("Transaction amount too small")};
        }
        txNew.vout.push_back(txout);
    }
    
    const CAmount not_input_fees = coin_selection_params.m_effective_feerate.GetFee(coin_selection_params.m_subtract_fee_outputs ? 0 : coin_selection_params.tx_noinputs_size);
    CAmount selection_target = recipients_sum + not_input_fees;
    
    
    if (selection_target == 0 && !coin_control.HasSelected()) {
        return util::Error{_("Transaction requires one destination of non-0 value, a non-0 feerate, or a pre-selected input")};
    }