Can I add a preview of a variable?
已回答

Like this, I want to load the value of a constant defined in another json file and display it in the code editor.
Is there any way to do this?
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This "preview" is generated by com.intellij.lang.folding.FoldingBuilder API which allows to replace original editor text, in this case with resolve target actual value. See https://plugins.jetbrains.com/docs/intellij/folding-builder.html