Online left-wing infighting seems to me to be about applying labels to people because they argue or have argued one thing on a particular topic, and then use it to discredit an unrelated argument topic or paint their overall character. I know there are pot-stirring trolls and compulsive contrarians, but I do witness users I personally judge to have genuine convictions do this amongst each other.
Within US politics, CA Gov. Newsom is an illustrative example (plenty of examples exist too for other countries and around Lemmy/Fedi). I don't particularly like him, he has done things I think are good, some things I think are funny, something things I think are bad and some things I think are downright horrible. Yet I have encountered some users online who will say they can't ever applaud a move of his if one specific other policy or set of other unrelated policies crossed a line for them. I'm not asking people to change their mind on what they think of a person because of an isolated good thing they do, but to at least acknowledge it as a good thing or add nuance describing what about it you like or don't. I can accept saying "I don't think this is a good thing in this circumstance", "this person will not follow through with this thing I think is good thing because ___", or "they are doing a good thing for wrong and selfish reasons" too. But to outright deny any support for an action because of a wildly extrapolated character judgement of the person doing it, when that user would support it otherwise, vexes me greatly.
I know this is not every or most interactions on Lemmy, but these are just some thoughts I have to get out of my head. You don't have to agree with me. I'm using 'left-wing' because the definition of 'leftist' or 'liberal' is wide-ranging depending on who you talk to. And on the side of the spectrum I'm calling left to left-centre, we seem to let the fewer things we disagree with get in the way of the many more things we would agree with each other. That's all, thanks for reading.