Is it browser's limitation?

I had issues, when tried to search in a very long <textarea> with scrollbar. Firefox was able to find only currently visible occurrences. That was not a viewport issue, of course, but looks very similar. Would love to know more about all this.

No, it's because Twitter Web removes elements from the DOM when they are far from the viewport.

There are two ways you can see it:

1. Open a DOM debugger, select an element in a tweet, scroll down. Eventually the element disappears.

2. In Firefox, scroll down, right click, "Take a screenshot", "Complete page". You'll see tweet far from the viewport, as well as header and sidebars are invisible.

The motivation is probably to make it faster, but it's unclear to me why removing (and later re-adding) elements in Javascript would be more efficient than the browser internally dealing with it.

Could you elaborate on what's meant by viewport in this context as, like this, it's new to me. TIA

What's currently displayed in the browser window [0]

[0] https://developer.mozilla.org/en-US/docs/Glossary/Viewport