Use conditional formatting and a global variable or field to list the selected values.
The "checkbox" is really a button that either inserts or deletes the selected item from the list in the global variable or field.
You can use layout text for this check box with conditional formatting that hides the text (change font size to 500) when the item is not part of the list of values in the variable or field.
Here's a demo file: http://www.4shared.com/file/47D2cLni/CheckboxWScrollBar.html
Note: after posting, I double checked this file and found that it does not use the conditional formatting I described, but instead uses a calcualtion field to show which values are selected. The calculation expression used in this field could be used as a conditional format expression instead.
I checked the sample file. It is excellent. But the check filed, cSelectionCheckBox, is for each record. If multiple users work on same record, the cSelectionCheckBox will be unpected for each user, I suppose. Does the calculate field same as global field wich value can be different for differnt user at same time?
The calculation field is unstored. It evaluates differently for every user as it refers to the global field that lists the selected values. Thus, each user can build their own list of selected values.
And keep in mind that you don't actually need the cSelectionCheckBox field, you can replace it with layout text and a conditional format expression similar to that found in this calculation field to hide/reveal the layout text. (You can make layout text disappear by having a conditional format change the font size to 500.)
Thank you. It is very clear.