Steps to Reproduce

<zk>
    <listbox>
        <listitem><listcell label="Some text to copy"/><listcell label="Some text to copy"/><listcell label="Some text to copy"/></listitem>
        <listitem><listcell label="Some text to copy"/><listcell label="Some text to copy"/><listcell label="Some text to copy"/></listitem>
        <listitem><listcell label="Some text to copy"/><listcell label="Some text to copy"/><listcell label="Some text to copy"/></listitem>
    </listbox>
</zk>

1. Select/highlight text using the mouse.
   1. select text within a single row OR
   2. select text over multiple rows
2. Press Ctrl-C to copy the highlighted text to clipboard.

Current Result

varying browser behavior
Chrome:

• single row: when holding CTRL the selection disappears (when releasing CTRL the selection is restored) -> making it impossible to complete CTRL-C to copy
• multiple rows: works in Chrome

FireFox:

• the selection is immediately removed after releasing the mouse

Expected Result

allow copying of text contents in a listbox
consistent browser behavior
 

Debug Information

https://github.com/zkoss/zk/blob/v8.6.2/zul/src/archive/web/js/zul/sel/SelectWidget.js#L606-L652

The code clearly disables selection on keydown event, without giving a reason in the comments.

Workaround

for chrome (ignore control/meta/alt key in the keydown method):

http://zkfiddle.org/sample/3a2ddud/2-user-select-none-alt-tab

all browsers:
using <listbox nonselectableTabs="*"/> doesn't interfere with copy/paste any more