In addition to having the form object set to Grow on a resize, you must also enable a selector for the 4D Live Window to be allowed to resize with the form.

The following code will allow the 4D Live Window to be resized with the form.
$error_i:=Web_SetPreferences (Web_HTMLArea_i;Web_kResize;1;"")

The following code will not allow the 4D Live Window to be resized with the form.
$error_i:=Web_SetPreferences (Web_HTMLArea_i;Web_kResize;0;"")

Notes:
Mac OS only: if the plugin area is resized, some web sites (specially maps.google.com) acts on the resize by using Javascript to change the position of some elements. This may lead to a crash if the site was already displayed. To avoid this crash the developer is supposed to set the browser area to a fixed size (not resizeable).