From https://github.com/jbosstools/jbosstools-freemarker/issues/26

When using the plugin to edit freemarker files that use the square bracket syntax the editor fails to highlight the syntax (this is happening in JBoss Tools 4.20 Final).

I think the file: src / org / jboss / ide / eclipse / freemarker / editor / DocumentProvider.java
on line 70 is causing the syntax highlighting problem:

if (ch != LexicalConstants.SQUARE_SYNTAX_MARKER.charAt(i)) { 
        return SyntaxMode.ANGLE; 
}

SQUARE_SYNTAX_MARKER.charAt(i) 

It should start in 0 and have a different index than i (the index of the file content) to have a proper string matching. Also, SQUARE_SYNTAX_MARKER is [#ftl, not all files start with a ftl tag, so I don't see the need for "ftl" at the end, so that's why in the following example fix I put j < 2.
e.g.

int j =0;
for (; i < docLength && j < 2; i++) { 
    char ch = document.getChar(i); 
    if (ch != LexicalConstants.SQUARE_SYNTAX_MARKER.charAt(j)) { 
        return SyntaxMode.ANGLE; 
    } 
       j++;
}