When using $wgCaptchaTriggers['edit'] = true;, the CAPTCHA field is marked as required, which means that the field has to be filled in even when trying to view the preview or the diff, even though the input won't even be validated in those cases. (You can test this at https://patchdemo.wmflabs.org/wikis/5f6311aecd/w/index.php?title=Main_Page&action=edit.)
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This can be achieved by using https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/submit#formnovalidate on the buttons.
Change 945854 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):
[oojs/ui@master] ButtonInputWidget: Add support for 'formNoValidate' config option
Change 945855 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):
[mediawiki/core@master] EditPage: Allow previewing even when the form is in invalid state
Change 945854 merged by jenkins-bot:
[oojs/ui@master] ButtonInputWidget: Add support for 'formNoValidate' config option
Change 953719 had a related patch set uploaded (by Catrope; author: Catrope):
[mediawiki/core@master] Update OOUI to v0.48.0
Change 945855 merged by jenkins-bot:
[mediawiki/core@master] EditPage: Allow previewing even when the form is in invalid state