Revisit signing flow and UX in the presence of sighash flags about app-bitcoin-new

[kewde]

To add a bit more context, all the browser extensions that I have interacted with circumvent this check completely by adding external dummy inputs if all the inputs their sighashes permit it. One typical use-case is listing an Ordinal NFT for sale.

app-bitcoin-new/src/handler/sign_psbt.c

Lines 1341 to 1346 in ea2e273

	if (st->inputs_total_amount < st->outputs.total_amount) {
	PRINTF("Negative fee is invalid\n");
	// negative fee transaction is invalid
	SEND_SW(dc, SW_INCORRECT_DATA);
	return false;
	}

While it certainly doesn't answer the questions you posed w.r.t UX, one quick win could be changing the non negative fee check to only run if all inputs use a default sighash (SIGHASH_ALL/SIGHASH_DEFAULT).