I think its a little more complicated when I tested it.
Sub-Applications are selected on mouse Down. Modules are selected on mouse Up. For SubModules the submodule node in the tree gets instantly selected when you do mouse Down. We then wait for few 100 millisecs and then active the submodule. Reason is that when you quickly move in a submodule tree using key cursor, its possible to jump over views without activating them.
I think for trees that is just the way tree's do selection. Its the behaviour of the native widget, that the tree node is selected on mouseDown. Dont you think ?
We'd prefer reaction on mouseUp both on Sub-Applications and Submodules (we don't have deep tree structures there).
I tried to verify how other applications (mainly MS- e.g. Explorer, Outlook) handle this, but well... they do it inconsistently as well )
Reaction (selected item content shown) mostly happens on mouseUp. But in Explorer selection happens even if I cancel selection with ESC. But only if I didn't start drag&drop. With D&D even old selection remains the same.
In Outlook ESC cancels selection either way.
-> Putting all together I still would prefer reaction on mouseUp and the possibility to cancel it with ESC