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I saw sometimes と indicating a topic. Like in the sentence:
そんな既視感に戸惑いを感じていると、こちらを見つめる視線に気が付いた ( My attempt of tranlation is "I felt
confused by that deja vu feeling by noticing the line of sight looking in this
direction"). I think it have the 「そんな既視感に戸惑いを感じている」as the topic of the
sentence, and then say that the person had that feeling of deja vu by noticing
a line of sight. Is that it or did I got all wrong?

Anothe exemple sentence is: 僕らの存在こんなにも単純だと 笑いに来たんだ (beggining of the opening
"UNINSTALL" of the anime ぼくらの). My teatcher said it means "Came laughing of
the fact that our existence is simple this way" (tried to make a more literal
translation)

In this last case, it seems that the と have the same role as the particle に,
indicanting the element of which someone is laughing.
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2020-08-11T15:25:23.600
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2020-08-12T02:22:05.800
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question
particle-と object topic
Can と indicante the object or the topict too as well as the を and は particles do?
No, と is not a topic marker. The と in the first sentence just means
"when/while ～" or "～ and (then)". (See: [What is the function of と in verb-
ていると form](https://japanese.stackexchange.com/q/23727/5010)) This 既視感 refers
to something mentioned in the previous sentence (because there is そんな), and 視線
refers to a different event that is probably unrelated to the cause of the
既視感.

> I felt confused by that déjà vu feeling, when I (suddenly) noticed someone's
> eyes staring at me.

* * *

Your second "sentence" contains a typo, and is actually a **part** of a
sentence. The full sentence is:

> 《あの時最高のリアルが向こうから会いに来たの》 **は** 、「僕らの存在はこんなにも単純だ」と笑いに来たんだ。  
>  (punctuation added by me)

The topic of this sentence is a long nominalized clause (parentheses) and is
explicitly marked with the topic marker は (bold). The と after 単純だ is just a
quotative-と used with 笑う. The quote is enclosed in the brackets. Xと笑う means
"to laugh saying/thinking X".

A very literal translation is:

> As for (the fact) that "the best real" came over to see me at that time,
> (it) was that he ("the best real") came to laugh at me saying our existence
> is simple like this.

Taking [the whole context](http://j-lyric.net/artist/a04d013/l00aa41.html)
into consideration, this sentence can be interpreted like so:

> I remember the day "the ultimate reality" came over to see me; he came to
> laugh at me and teach me how trivial our existence is.

(Note that 最高の is a sarcasm here. Clearly, he does not want to accept the
ruthless reality that he is just a piece of dust in this universe.)
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answer