The Display settings dialog box can't be used without a mouse or trackpad, because the elements can't be focused and cycled using Tab. That's obviously an accessibility issue, as well as annoying when my laptop unsuspends with both a borked display resolution *and* a disabled trackpad! It does work as expected for other Settings dialogs.
Works for me with arrows (left and right) or tab, one just needs to press enter to it take effect. With regards to your resolution issue, maybe the latest (development) release could help: https://simon.shimmerproject.org/2018/10/02/new-xfce4-settings-release/
Thanks for the pointer to the latest developments Andre; let's see! Strangely I'm still unable to get Tab focus-switching to work. Pressing Enter opens the display selector drop-down, but I can't make it switch to other elements. The equivalent thing happens if I select another element with the pointer: I can then open/close it with Enter, but not keyboard-switch to other elements. This is in an up to date Xubuntu 18.04. As mentioned it works for other dialogs so not a general glitch in my system's focus selection.
-- GitLab Migration Automatic Message -- This bug has been migrated to xfce.org's GitLab instance and has been closed from further activity. You can subscribe and participate further through the new bug through this link to our GitLab instance: https://gitlab.xfce.org/xfce/xfce4-settings/-/issues/119. Please create an account or use an existing account on one of our supported OAuth providers. If you want to fork to submit patches and merge requests please continue reading here: https://docs.xfce.org/contribute/dev/git/start#gitlab_forks_and_merge_requests Also feel free to reach out to us on the mailing list https://mail.xfce.org/mailman/listinfo/xfce4-dev