Greta Thunberg has been arrested for terrorism in London for holding a sign
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I think you just covered both of them.
A direct threat would do it in most places. You could use the bomb word for example. You could throw in other enhancements as well if you wanted to make sure.
I think it’s important to consider what is on the sign as we render our judgements.
In Germany, a Nazi swastika is a criminal symbol because Nazism is considered an existential threat since Germany's constitution is built around "defensive democracy", or wehrhafte Demokratie. Consequently, the state is obligated to prevent the re-emergence of Nazism. If you display a Nazi swastika, you will likely be detained for prosecution under § 86a StGB.
There are only a few exceptions (art, science/education, journalism, explicit anti-Nazi protest), but the context has to unambiguous.
This is different from "terrorism law"; instead criminalization of the Nazi swastika is protecting the constitutional order.