However, this cannot be the right fix: with this patch the editable
field widget is no longer editable (it also doesnt't extended to the
right window margin, and it's one space shorter than when the
widget-field face is edited to remove the :extend property (without the
patch)).  Moreover, in emacs-27 the editable field is also extended yet
there's no display glitch.  (There's one difference in
widget-specify-field between emacs-27 and 28/master, but undoing that
change in the latter does not eliminate the display glitch.)

There's also something strange about customizing the widget-field face:
when I uncheck the box for "Extend" and click the "State" button to "Set
for Current Session", nothing happens with -Q and without -Q I get the
message "End of file during parsing".  If (without -Q) I click "State"
to "Save for Future Sessions", then it says "SAVED and set" but the
"Extend" checkbox remains checked and the editable fields are extended.
If I keep "Extend" checked and click the Value Menu to "Off", then I can
set the change for the current session, and then the editable field
widget is not extended, but the display glitch remains (and the editable
field is one space longer than with the above patch).

Steve Berman