The content of the question reminds me of when we used to use images to create rounded corners on websites before CSS3.

The crazy thing is how obsessed we were with bizarre page layouts that necessitated these hacks. People had fewer tools and also designed crazier websites. They were always difficult.

But the real answer to the question for me are any times when the code needs to be in tight coordination with deployment as to not break anything. Meaning caches or data typically. Those are the toughest challenges I have to deal with.

Anything visual is hard for me. I can't "create" layouts in my mind, just ideas, concepts, raw text. And that is though, because I really need a new portfolio site and I spend hours thinking just on content and not on how to make it stunning.

Recently in my job, the most difficult thing I had to do it's manage a background process to download invoices, extract the data, and show to user in real time the extracted data, and then he can replace some data, and if he send the changes, now in real time everyone can see the changes of that invoice and then in background again process every relation with that invoice and change it. (Everything was using WebSockets & Channels in Go)

I decided to make a project called "can I lick it." It is basically where you can enter the name, number, or symbol of an element on the periodic table, and then it would tell you if it is safe to lick, as well as some information about it. Right now, I am around 60 elements in. Rather than just using an API for the data, I decided to put all of it in JSON. I have taken a break as this has been a long boring process, but I feel to deep to stop.

Like everything we do... it just takes practice.
If you've built one website a year... you're not gonna get good at it.
You need to pump out multiple layouts and multi-templated sites that are responsive if you hope to get good at doing responsive frontend development.
There are very few devs who are really good at that.

A LOT of devs can build some fancy thing on desktop, but then can't make it responsive.
And plenty of devs can do responsive, but can't handle complex animations, or code them in a way that lasts for a few years and doesn't break due to updates in the libraries and scripts they use.

Nothing we do is hard. It just takes time to learn. And once you learn it, you won't why it was ever hard... if only someone had given you the steps to properly do it.

That being said, the most difficult things i've had to code.
Not sure, I've built custom calculators, built design tools, built image manipulation tools, built a social network, built a gaming site, built a site crawler, built my own caching stuff.
Some of it was hard at the time, but only because it required effort. Once i found the solution, looking back, it wasn't hard to do.

Knowing is half the battle

Nice question—it really made me think. I recalled a time when I was developing a microservice using Java Spring Boot. I found it challenging to create singleton objects using Spring's annotations. Initially, I had to resort to writing my own singleton classes to achieve the desired behavior.

Honestly, one of my old school assignments comes to mind; we had to implement tcp communication with c++(I started on that first minute but still only barely finished in time before it was due)
My answer these days would be front end related stuff since there's so many quirks and unexpected things happening

My first FAST API structure damnn

Getting a complex dynamic form system to look good on every device nearly broke me, honestly UI stuff is always the hardest part for me too.

Did you ever try using CSS grids for your layout?

Maybe this game: dev.to/pengeszikra/javascript-grea...
Because this use a single JS function to able use JSX without framework. Have a reactive state handling system which is don't belong to view. Power of that state handling system is show the card game part which can controlled by player or code also - easy switch. Contain a CSS 3D system. Contain a real time markdown view webComponent including some basic syntax highlight for code block. Whole app compiled less than 200k. Responsive and run on any device.
Have own sci-fi world and created a AI short movie of the contact the flogons and humans. Protagonist is a fictive punk-metal singer, she have 13 tracks on my youtube channel.