Looks like we can't reproduce the issue or there's a problem in the test case provided.
Non standard grid selection
In most GUI toolkits I could try (Windows, Swing, Qt, GTK+...), except, I admit, SWT (strangely), the list / grid component has a quite uniform behavior with regard to selection.
Classical: click on a line to select it, shift-click on another to select between the two (included).
Additional: Ctrl+click on rows to toggle their selected state.
Less known: Ctrl+click on a row, then Ctrl+Shift+click on another to add the new range to the existing selection.
Here, exact behavior change between toolkits: in Windows Explorer and any native Windows list component, this still act as a toggle (which is the most useful): if you Ctrl+click in the middle of a selection, then Ctrl+Shift+click below, still in the selection, the defined range is unselected (except the last clicked row). In most other toolkits, it just add to the selection (ie. does nothing).
You can see this behavior, for example, in the Advanced Search of this forum, in the list of forum sections, the list acts this way (at least in Firefox, which uses its own toolkit, I think).
The GXT 3 Grid doesn't act like this... Ctrl+Shift+click just toggle the row, like a simple Ctrl+click. So I open this bug / change request to improve the grid and allow such handy range selection shortcut.
Its even more complex than this. Some actions act already on a mousedown, while others require the full click.
I've filed a ticket against the team to review the semantics of modifier keys and grid selection. This is something that deserves consideration. With the bugs that remain in the push toward 3.0, there may be some time before the appropriate discussion can take place, but I'll update this thread with any changes we make.
Actually, in Eclipse, lists have the behavior I describe!* It doesn't work for tree siblings, though, that look like a list when there are lot of items, hence my confusion. I would say it is a bug in the tree implementation of SWT...
except, I admit, SWT (strangely)
* Examples: Ctrl+Shift+R -> Open Resource, Ctrl+Shift+T -> Open Type, Ctrl+Shift+E -> Switch to Editor, all these dialogs have multi-selection lists with this behavior.