Reading - Why popup dictionary doesn't appear in the Beginner tab and the Advanced tab?

Popup dictionary works in these two tabs absolutely. But unlike mouseover, you should click on a word for which you need dictionary to bring it up.

This is intentional. Because the Browser tab is designed for users to skim tons of Chinese information, so word definitions are accessed effortlessly. While the Beginner tab, Advanced tab, Examples tab, and HSK tab focus on deep reading, to reinforce your memory you should consult dictionary only until you can not recall or infer a word's definition. That is to make you read Chinese texts yourself with a bit of help when needed. We believe that continually looking up words while reading is detrimental to long-term learning objectives because it lets you fool yourself into thinking you understand them when you do not.