"Which chair is it?"
Yes with topic I mean a subject to talk about. If I use the word subject it is likely to be confused with the "subject" as in "object and subject of the verb". Sorry that it still seems to be confusing.
Anyway you use は if you want to specify which is the topic you are talking about. You can skip the whole element if it is already understood. I give another example: if you are discussing where to go for lunch with 2 friends, and you say パスタが好きですか you will be understood as asking both of them whether they like pasta. If you are only expecting Tanaka san to answer, you need to specify your topic is Tanaka san but not the other friend, you would say 田中さんは パスタが好きですか. If it is in the circumstances that all 3 of you understand that your question is for Tanaka san only (topic is Tanaka san), it is more natural that you skip 田中さんは. For example, while you are discussing, Tanaka san suggests first to try European food, you think of a good pasta restaurant and turn to her and ask パスタが好きですか, all of you understand that Tanaka is the topic of your question, i.e. the question is asking whether, about Tanaka this person, pasta is pleasing her or not.
Both the system and your dictionary are correct. The Kanji of どの is 何の but your dictionary should, if it is good enough, have told you it is usually written in Kanas.
何の written in Kanji usually refers to なんの, meaning "What kind of".
Also, I tried both my Samsung keyboard and Google Japanese keyboard. Both of them do not suggest 何の as an output when I type "との". I am not using Romaji input so you can try yours and test how good it is.