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I found a pretty cool plugin https://github.com/wellle/context.vim, which can show the out of screen header of current block like function, if, while, etc,. And I think treesitter may be more suitable to do this work. Since nvim-treesitter already has the feature like highlight current scope, showing the header of current scope when it is out of screen is also well-reasoned. More importantly, treesitter may be more efficient and suitable to implement that than regular expressions.
This is a useful feature when you face to a long function or if/else. A header showing on top of screen let you know what function or condition branch you are in. Of course, some plugins which can show function name on status line are also take same effect. But personally, I prefer top of screen instead of small space of status line. And in addition, the if/else/while/switch stuffs can also show together if needed.

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Hi ! Thanks for the suggestions !
Unfortunately, we tend to try to minimise the amount of modules this repo has, and we might not implement this in this repo.
However, this is an awesome idea to have in another module, and we might try (with your help if you want) to implement it in it's own module using module-template.
Hey ! I got a POC working here, as an nvim-treesitter module ! It is pretty rough for now, but I might try to add some things to it.
EDIT: Here is the link : https://github.com/vigoux/treesitter-context.nvim
I am closing this now.
If anyone stumbles here, here is an implementation with highlighting: https://github.com/romgrk/nvim-treesitter-context
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If anyone stumbles here, here is an implementation with highlighting: https://github.com/romgrk/nvim-treesitter-context