Feature summary (what you would like to be able to do and where):
Allow the selectandother field from HTMLForm to allow the "required" attribute be configurable to instead require that either one dropdown option (other than "other") is picked or text is entered into the input. Currently setting required to be true only makes the input required.
Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
On Special:InvestigateBlock and also Special:CheckUser when in 'get users' mode, the block forms allow the user to enter in a required block reason. Being able to add a dropdown reason, like Special:Block has, would improve speed of using the block forms in CheckUser / Investigate, but the field must still be required due to the design requirements so that the block reason would not be empty. In cases like {{CheckUserBlock}} on a IP, linking to the case or user is against policy so there would be multiple occasions that the desired outcome should not have any text in the "other" input field.
Benefits (why should this be implemented?):
This would allow the selectandother field to be used in Special:InvestigateBlock, as the validation seems to be done just on the client side. This would improve Special:CheckUser as a current patch in review relies on a error returned by the server after submitting the form which is not user friendly.