Called before the buffer's maximum or minimum height or width is changed. If the return value is #f, then the change will be aborted. Otherwise, after-set-size-constraintis guaranteed to be called after the change has completed.
(This callback method is provided because setting a buffer's maxmimum width may cause lines to be re-flowed with soft carriage returns.)
See also on-edit-sequence.
Dispatches to the mode.