Hi, that was a tricky one, and had me scratching my head for a bit. :-)

First, I don't know how the line

$win=new Gtk2::MainWindow();
[download]

ran for you. It should be

$win= Gtk2::Window->new();
[download]

that was easy. :-) But now to the hard problem.

I got a clue from how the TextView scrolled to end by needing a signal connect on it's underlying buffer, which without it, would exhibit the exact same symptoms of the scroll-to-end going off target after the is window resized smaller. So here is a working version of your script, notice the $model->signal_connect( 'row-inserted' => sub{}).

#!/usr/bin/perl
use strict;
use warnings;
use Glib qw(TRUE FALSE);
use Gtk2 -init;

my($win,$tree)=createWin();
Glib::Timeout->add(1000,sub {tickCB($tree)});

$win->show_all();
Gtk2->main();


#
# Creates the widgets in the application.  Returns the main
# window and tree view.
#

my $model;

sub createWin
{
   my($win,$scroll,$tree); #,$model);

   #$win=new Gtk2::MainWindow();
   $win = Gtk2::Window->new ();
   
   $win->set_default_size(250,300);
   $win->signal_connect(destroy => \&Gtk2::main_quit);

   $win->add($scroll=new Gtk2::ScrolledWindow());
   $scroll->add($tree=new Gtk2::TreeView());
   $tree->set_rules_hint(TRUE);

   $tree->insert_column_with_attributes(-1,'Goo',
      new Gtk2::CellRendererText(),text => 0);
   $tree->set_model($model=
      new Gtk2::ListStore('Glib::String'));


# without this signal connect, making the window bigger
# will be ok for scrolling, but will screw up
# on making window smaller
   
   $model->signal_connect(
      'row-inserted' => sub {
          my ( $view, $iter, $path ) = @_;
         # print "@_\n";

         my($numRows)=$tree->get_model()->iter_n_children(undef);

        $tree->scroll_to_cell(
         new Gtk2::TreePath($numRows-1),undef,TRUE,0.0,1.0);
   }
);


   addWord($model) for 0 .. 100;
   showLast($tree);

   return ($win,$tree);
}


#
# Called at regular intervals to add another random "word"
# to the bottom of the tree view.  If the previous word was
# visible beforehand, scrolls the tree view so the new word
# is visible.
#
sub tickCB
{
   my($tree)=@_;
   my($model)=$tree->get_model();
   my($numRows)=$model->iter_n_children(undef);
   my($lastVis)=($tree->get_visible_range())[1];
   my($mustScroll)=$lastVis &&
      $lastVis->get_indices() == $numRows-1;


   addWord($model);
   showLast($tree) if $mustScroll;

   return TRUE;
}


#
# Adds a random "word" to the bottom of the tree view.
#
sub addWord
{
   my($model)=@_;
   my(@cons)=grep !/[aeiou]/,'a' .. 'z';

   $model->set($model->append(),0,
      $cons[rand @cons] . 'oo');
}


#
# Scrolls the tree view so the last row is visible.
#
sub showLast
{
   my($tree)=@_;

   my($numRows)=$tree->get_model()->iter_n_children(undef);

   $tree->scroll_to_cell(
      new Gtk2::TreePath($numRows-1),undef,TRUE,0.0,1.0);

   $tree->set( vadjustment => $tree->get_vadjustment );

}
[download]

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