When an item is too long to display in the current view, the TreeView correctly pops up a tooltip to show the missing item text. However, this currently pops up behind the form which hosts the control, which isn’t particularly useful.
I had exactly the same problem with a superclassed treeview control I wrote a while back. I got around the problem by explicitly bringing the tip window to the top of the z-order whenever it was shown; I do this everytime I get a TVN_GETINFOTIP msg, but I expect it would work fine if you just did it as part of the control initialisation if you're not using TVS_INFOTIP.
// get tooltip control
HWND hWndTooltip = (HWND)SendMessage(TVM_GETTOOLTIPS);
// make sure it is topmost
0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE | SWP_NOACTIVATE);
A fix has been documented for this by Microsoft. See KB241102.