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One way to ensure reliable rehighlighting of multiline Font Lock constructs is to put on them the text property font-lock-multiline. It should be present and non-nil for text that is part of a multiline construct.

When Font Lock is about to highlight a range of text, it first extends the boundaries of the range as necessary so that they do not fall within text marked with the font-lock-multiline property. Then it removes any font-lock-multiline properties from the range, and highlights it. The highlighting specification (mostly font-lock-keywords) must reinstall this property each time, whenever it is appropriate.

Warning: don't use the font-lock-multiline property on large ranges of text, because that will make rehighlighting slow.

— Variable: font-lock-multiline

If the font-lock-multiline variable is set to t, Font Lock will try to add the font-lock-multiline property automatically on multiline constructs. This is not a universal solution, however, since it slows down Font Lock somewhat. It can miss some multiline constructs, or make the property larger or smaller than necessary.

For elements whose matcher is a function, the function should ensure that submatch 0 covers the whole relevant multiline construct, even if only a small subpart will be highlighted. It is often just as easy to add the font-lock-multiline property by hand.

The font-lock-multiline property is meant to ensure proper refontification; it does not automatically identify new multiline constructs. Identifying them requires that Font Lock mode operate on large enough chunks at a time. This will happen by accident on many cases, which may give the impression that multiline constructs magically work. If you set the font-lock-multiline variable non-nil, this impression will be even stronger, since the highlighting of those constructs which are found will be properly updated from then on. But that does not work reliably.

To find multiline constructs reliably, you must either manually place the font-lock-multiline property on the text before Font Lock mode looks at it, or use font-lock-fontify-region-function.