If you add a word to the custom dictionary (by clicking Learn Spelling), words with apostrophes won't add. They will remain underlined in red. For example:
structuration (flagged as a misspelling, learned, and no longer flagged)
structuration's (flagged as a misspelling, clicked to learn, remains flagged)
Any chance you'll fix the annoyance, which has existed since 1.0, that two words separated by an em dash are flagged as one misspelled word? For example, "get—but".