From J Wiki
The following script modifies the behavior of viewmat verb so instead of creating a new window every time it is called, it reuses an already open window. This is similar to how plot acts.
Works for J602, probably will not work for J80x/qt
NB. J602, hack to "reuse" viewmat window NB. 2015-01-07 16:54:43 created by nsg viewmat_z_=:3 : 0 '' viewmat y : for_l. conl 1 do. if. 0=nc__l <'COCREATOR' do. if. COCREATOR__l-:<'jviewmat' do. NB. !!! FOUND jviewmat form wd 'psel ',HWNDP__l x viewmatcc__l y;GID__l glpaint__l '' return. end. end. end. x viewmat_jviewmat_ y )