In most Linux environments "alt + mouse drag" moves the current window and the event is never delivered to the application. This is the only way I know of selecting source code in Crucible, so I usually end up either memorising the contents or opening the same file in IDE in order to perform copy paste. Ideally there would either be an additional keyboard shortcut (e.g. alt + shift + mouse drag) or a toggle button in the UI for being able to select text. Toggle button might be better as it would be more discoverable.