But I may also be wrong.
Anyway, there are many other formal contexts where Díky is used. It is hard to say where it is a noun and where an interjection. Basically, the informal connotation only refers to the use as the sole word. In many other phrases or sentences it can be formal.
A hover is available for the full phrase. Deviating from it in favor of a single-word hover lower in the window is usually at the user's own risk. Dobrý večer is not not a parting greeting, just as dobrou noc is not what works when you are just meeting. These greetings are directional, so to speak, one for coming, the other for going.
Aha! Now that makes sense. I'm beginning to see why Czech took so long to complete! This is sounds like (British) English in saying "Good Morning" (afternoon, evening) when meeting, the inflection is on the "morn". When we take our leave the inflection is on the "good". Very slight difference, and even many English people miss it.
In fact, you are right. Phonetically, i/y is [ɪ] and í/ý is [iː], not [ɪː] as one would expect. But most native speakers are not aware of the quality difference between so called short and long i/y, they think the only difference is the vowel length.
When listening carefully, it is possible to hear the difference - the "í/ý" sounds softer and more closed. Also when trying to simply prolong the "i/y", the final sound is not the same as when pronouncing "í/ý".
Well, I don't know what bbrukernavn had in mind, but for example certain endings have i for masculine animate gender and y for others.
In Common Czech you often have ej instead of ý (dobrej, zejtra), but less likely for í (a result of a sound change in spoken Czech ý => ej in the 15th(?) century which was not accepted into literary Czech).
In the 14th century i and y did differ and they still do in Polish and Russian.
And there's something else I want to tell you. Czech is said to be one of the most complex languages for foreigners. In Czech, differents in "i, í, y, ý" is very hard. And it hard for Czechs, too. Our language is said to be one of the most complex languages for foreigners.
Actually, the i/y diffeence is more problematic for Czech schoolchildren than it is for foreigners. From observing the questiins and reports of people taking this course, the i/y spelling is not a big problem at all. The morphology and the word order, on the other hand...
Many languages have the same complexity as Czech or even higher, the "most complex" is a myth.