I am terrible sorry guys, and especially you @chris.
It turns out to be a little glitch when changing the Item list box fill a year ago. A parameter was missing in the new one.
The function in ui_lists.inc, line 776:
function stock_items_list_cells($label, $name, $selected_id=null, $all_option=false,
$submit_on_change=false, $all=false, $editkey = false)
indicates a final parameter $editkey. If this is true, a f4 key is available to call upp the items form in a popup window.
So in file /purchasing/includes/ui/po_ui.inc line 401:
stock_items_list_cells(null, 'stock_id', null, false, true, false);
should have a last parameter set to true
stock_items_list_cells(null, 'stock_id', null, false, true, false, true);
And finally there was a little bug in /inventory/manage/items.php on line 22. Should be:
page(_($help_context = "Items"), @$_REQUEST['popup'], false, "", $js);
Try to change theese lines and it will work with the f4 key. Some times you have to pres this key twice.
So F2 = customer/supplier, F3 = customer branches and F4 = items in forms where this is established.
This will be sent to the HG repository. First I will have a look into other scripts if it need changing.
You can also download the 2 files here. Rename the items.php1 to items.php and replace the files
/inventory/manage/items.php
/purchasing/includes/ui/po_ui.inc
/Joe