different sites behave differently, but the popup is generally just a bit too eager with selecting text.
for example, in syosetu it's hardly noticable: if a character at the end of a line has furigana there's a little bit of space to the right of it which causes a word in the next line to be selected
other sites are much worse. for example, in the youtube comments section, if you have your mouse anywhere inside the dotted lines here, it will select whatever text comes after your mouse. this is very annoying since I have to bring my mouse all the way outside of the text box to get it to stop defining words instead of just moving into some whitespace

different sites behave differently, but the popup is generally just a bit too eager with selecting text.
for example, in syosetu it's hardly noticable: if a character at the end of a line has furigana there's a little bit of space to the right of it which causes a word in the next line to be selected
other sites are much worse. for example, in the youtube comments section, if you have your mouse anywhere inside the dotted lines here, it will select whatever text comes after your mouse. this is very annoying since I have to bring my mouse all the way outside of the text box to get it to stop defining words instead of just moving into some whitespace