There were no unit tests for this whole module. It is difficult to test
due to all the private logic and Qt dependencies, but with a lot of
mocking we can at least validate some of the text handling.
This is a setup to start testing the solution to #4199.
I picked '{' and '}' as placeholders in the test data because they draw
the eye to the 'highlighted' part, and vim even highlights them with
python syntax highlighting. It could be confusing though, as they look
like format strings but are not used that way.
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