The default font size is set to 12, and then when opening the power management in the settings, the settings will freeze. If the font size is set to 11 or 13, it won't freeze. Changing the font to another font will also cause the default font size to freeze.
I can't reproduce this as described (though the description is a bit thin; maybe you can attach a screen recording?). However it does appear that *something* is going wrong with this KCM when opened with non-default fonts; see Bug 499773 and Bug 499991.
Created attachment 179195 [details] Turning on power management in Settings after modifying the default font causes the Settings app to freeze
Thanks for the recording! It looks like this is the combination of font size and locale that also comes up in bug 499991 - merging this into that bug report. Thanks! *** This bug has been marked as a duplicate of bug 499991 ***
*** This bug has been marked as a duplicate of bug 499773 ***