[fr] Application wide font size variation as common in browsers, key combination control plus/minus. This works on named fonts, especially the default widget fonts. Minimum value is used to avoid reversing effect on negative size values.

 proc fontresize {i} {
   if {$i>0} {
     foreach f [font names] {
       set fs [font configure $f -size]
       if {[expr {$fs>20}]} {
         return -code break
       }
     }
   } else {
     foreach f [font names] {
       set fs [font configure $f -size]
       if {[expr {$fs<3}]} {
         return -code break
       }
     }
   }
   foreach f [font names] {
     set fs [font configure $f -size]
     incr fs $i
     font configure $f -size $fs
   }
 }
 proc test {} {
   package require Tk
   bind . "<Control-plus>" {fontresize 1}
   bind . "<Control-minus>" {fontresize -1}
   entry .e -textvariable ::z
   set ::z input
   label .l -text label
   text .t -height 2 -width 40 
   .t insert end "press control-+ or control-minus\nto adjust global font size"
   pack .e .l .t
   toplevel .k
   label .k.l -text another\ child-window
   listbox .k.f -listvariable ::f
   set ::f [font names]
   pack .k.l .k.f
   focus .
 }
 test

----
[JH] Note that tklib style::as provides this via '''style::as::enable control-mousewheel local|global'''

----
!!!!!!
%| category GUI |%
!!!!!!