When marking something to copy and paste. You start marking the text and drag to the right. If you drag too far to the right, your highlighting goes away and everything to the left of where you started becomes highlighted. Why would anybody ever want that behaviour? It is exactly the opposite of what you are trying to do.
Oh yeah. Or trying to highlight something including text further down, you want to scroll it a little bit and suddenly it accelerates and you highlighted the wohle page.
This is one of those things that came out of the gate sucking massively and nothing was ever done to improve it. We might be in our 60s some day still fumbling around trying to copy some text.
The browser implements the text selection behaviour, but how infuriating it is depends on how convoluted your page construction is.
On a simple page with no floats, overlaid elements, negative margins, absolute positioning, hidden stuff, and other css layout tomfoolery, it’s perfectly predictable. It’s only when designers do designer things does it start to break down.
A lot of these techniques aren't really used any more. We're old lol. Modern web design uses CSS grid, or at least Flexbox. I haven't seen a float or absolute positioning in years.