Navigation and punctuation should trigger spell check
If "Spell check as you type" is on, typing a space -- or pasting in text that contains spaces -- triggers a wavy red underscore under any misspelled word that's followed by a space.
However, if you type a word in the middle of some text (i.e., editing existing text rather than typing some new text) and don't type a space, then navigate away using arrows or home/end or pg up/dn or the mouse, no red underscore is triggered. This is a bug.
Also, if you type misspelled word followed by a punctuation mark and no space, this too fails to generate an underscore but should. Strangely, if you paste in such a sequence, an underscore is triggered, as it should be.
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Perhaps you would like to search Bugzilla with your observation and subscribe to them to follow future discussions, because filing multiple support requests as you are doing is going to achieve nothing in the long run as some of these are issues have been around for 20 years.