Explain that working from home is WORKING

I think you've had aome good suggestions here OP on using metaphors to explain it to him. But I thibk at this point a frank discussion of 'you cannot come in my office between these hours. If I have popped out for a break I can manage a quick chat but otherwise I absolutely cannot be disturbed'
If he still disturbs you i honestly would think he's doing it maliciously as there is no reasonable excuse

You've been together 10 years and have only just started living together. Conveniently timed to coincide with him not being able to work due to an injury.
He won't be going back to work again OP, now he's living in your place and you're working to bring money in for both of you.
All very convenient.

He's completely convinced he'll be able to go back to work and that this is all just a temporary phase

He's talking the talk. That's what he thinks you want to hear.

Brace yourself for this being a permanent situation with him not working until retirement.

You tell him your working hours and tell him he is not to disturb you. Put a lock on the door if necessary and ignore him completely during your working hours. If he doesn't like it, he's free to move out again and find a job to pay for it

Tell him if he can’t respect your work enough to leave you in peace, he should move out again.

@wfhorwtf, my guess is you got cross today because he hasn’t got the message yet. And the reasons he hasn’t got the message yet are a mixture of his personality, his current circumstances, and the fact that as a reasonable person, you haven’t been as firm as it turns out you need to be every single time since he moved in.

After today’s working day is done, you can re-establish the boundaries.

These things can creep up from relatively small differences in approach, and aren’t impossible to resolve, but will need him to listen to you and respect that you effectively are unavailable during working hours.

I couldn't have a relationship with someone so incredibly stupid.

Why do you keep reminding him, he doesn’t listen! Lock the door.

Agreed. @wfhorwtf is either seeing him through deeply rose-tinted glasses, and he is in fact thick and/or controlling, or his reasoning abilities and/or theory of mind have been damaged.

"I need to concentrate"
Never mind explaining algorithms to him.

It's canny insulting to tradies to imply, because he's a tradie and worked manual jobs he can't understand not to interrupt someone when they're working. It's not because "he's removed from that world" it's because he's a nob.

OP you sound afraid of him or at least afraid of upsetting him.
Why can’t you just tell him to F off out of your office? He is undermining you and your job and you keep making excuses for him.
I WFH and everyone - including children - know not to disturb me. For any reason other than the house burning down. Regardless of whether they can hear me on a call or not. If a child can understand this so can your partner.

Just wanted to say that you sound like a lovely, patient and tolerant person and I hope using 'pulling off the road to watch a 10 second video then having to wait for a gap in traffic and get back up to cruising speed' analogies help.

I also thought the observation that his accident means he's understandably probably a bit needy now was astute.

He doesn't sound like a bad man.

It would do my head in though too. Sympathy.

I had a bit of this when working from home. Virtual call centre stuff involving benefits and money and then notes recorded on a case management system and follow up email done.

Herself struggled in particular with not interrupting during write up time and that if I'm 'available' on work's call managing system I can't scoot downstairs to make, or even fetch/carry cups of tea. Missed calls happen now and then but too many and there'll be an inquiry. Neither is at acceptable to go into pre or post call modes to avoid incomings.

However I retired yesterday so it won't be a problem in future.

I think maybe he needs to go back to the GP and ask for advice re cognitive impairment. This makes no sense otherwise.

This is about far more than a partner interfering with work time.
I knew someone who had been in a serious accident as a HGV driver and his mental health really suffered as well as flashbacks and nightmares.
Have you talked about the future? Because if he can’t work again you are going to have more problems than this.
Good HGV drivers are really skilled people who can multi-task.

Either that or does actually get it and he’s just completely disrespectful and even though she has explained it to him, he thinks his desire to speak to her is more important than her need/wish to get on with her work.

I know you keep saying he isn’t stupid but it sounds as though he really cannot see anything outside of his own experience and understanding. Someone who really believes that your full time job is the same as “scrolling social media and ignoring him” just because you’re at a computer doing work he doesn’t understand is… well, is just a bit stupid.

My ex used to do this. What's happening is you're being reminded, subtly but repeatedly, that you don't matter. Your career is less important than some stupid youtube video. It's very much a "boiling the frog" effect, but over a long period of time, it will wear away at you.

Honestly I'm a bit suspicious of this "can't understand that it's work" thing. Initially, fair enough, but after being told several times then it's not a failure of understanding, it's just thoughtlessness. I guess if you engage with him sometimes (when you're getting a cup of tea or something) it might have reinforced the idea that you're open to being communicated with.
In terms of practical advice, it sounds like you're better at abstract thought than he is (not slagging him off, people have different cognitive styles). So you probably need to use a metaphor - what if he was trying to build a wall and you came along and knocked his bricks on the floor every time he picked them up. Something like that.
A final thought - coding requires more analytical intelligence than most trades - do you think he feels a bit threatened by this and needs to demean your job as not real work.
If all the above fails, he's a cock and you should get rid.

I wouldn't get into playing around like that, just very clearly, you are not to disturb me, I am working, you may not get it, but you don't have to, this is my career, please stop interrupting. Also, considering you've been together nearly a decade, I would also be adding that I don't care about his ex's social media addiction from ten years ago, and to stop comparing me to them for doing my job, as it has nothing to do with what he is doing, interrupting your work.

I find it strange that he seems to have no kind of understanding or respect for your work- are you sure there's no cognitive impairment?

Then suggest he sets up a workstation for himself, to keep up with the podcasts etc. Put him to work finding resources outside the home - chess clubs, lectures, discussion groups, libraries, museum tours??

I really don't feel a grown-up should need coaching/coaxing like this. I'm making a huge allowance for his radical change of circumstances.

On the whole I agree with everyone saying it boils down to disrespect; I'm trying to find a way through without offending your devotion to the bloke. But I think you must clarify your own thinking: if you allow his wants to damage your professional performance, you'll both suffer consequences.

He does understand that you're working (unless theres a drip feed that he's got some kind of mental incapacity), he just doesn't care about interrupting you.

I don't think that will work, @wfhorwtf
He wants your attention all the time and doesn't understand the principles of your job.
Could he go and do some volunteering?
The Salvation Army are busy at this time of year, also soup kitchens, community food hubs etc.
Maybe he needs to get out of the house.