I have found the original webcomic in Chinese: https://ac.qq.com/Comic/comicInfo/id/630166
Meaning Of "What if..."
I have taken a look at the webcomic in Chinese. From my perspective, this webcomic seems to be filled with sexual innuendos. And it is no surprise that the Tumblr website has "yaoi" in the URL. Wikipedia writes:
Yaoi, also known as boys' love or BL, is a genre of fictional media
originating in Japan that features homoerotic relationships between
male characters. It is typically created by women for women and is
distinct from homoerotic media marketed to gay male audiences, such as
bara, but it also attracts male readers.
That explains the sexual innuendos. Of course, not every comic appears overtly sexual; it may just focus on the homoerotic relationship.
The "What If" is a concern, because Zhan Xixi may be wondering what the other guy is doing in the bathtub after so long. What if he is doing this? What if he's doing that?
Why is Jian Yi an idiot?
The webcomic mentions "2 hours later" in the beginning. Zhan Xixi wonders what in the world the guy is doing in the bathroom. What if he is doing this? What if he's doing that? Heck, what if he's actually masturbating? When he goes in, he notices that he is still taking a shower. Imagine that. A person takes a shower for 2 hours and does not appear to be finished. Then, Zhan Xixi says: "你是白痴吗？!!!" In the Chinese language, 白痴 is one word that can be translated to "idiot" in English, but this word has the connotation that the person does not know how to live, does not know how to take care of himself, does not have interest in life. When the person does anything, he fails to do it properly. When the person speaks, he lacks social graces and is not a good conversationalist. In context, 白痴 is a curse word at Jian Yi, because Jian Yi should be finished with his shower by now. He is an "idiot" for not knowing how to take a shower, which is a basic life skill.