If you can provide a small test program that has this problem along with C source code I code try and see what is going wrong. FWIW I just put together, compiled and tested a small program using LISTBROWSER_Borderless and I didn't get any issue as you describe.
Ok, I've coded it up in C, and it has the same behaviour. You may not have noticed the border, so I have set the ListBrowser background to
white for every item. I can't find any way to get rid of the small grey border (or about 5 pixels), which surrounds it. This area acts as if it were
part of the ListBrowser in that clicking in it will select the node, but is actually the space is added in addition to the specificed column size.
I had expected LISTBROWSER_Borderless to remove that extra space.
Ir does not really look bad; you might ask why I care. What I am doing is trying to make a subclass of ListBrowser that will give it more of
a Spreadsheet type feel, and need to know which row and column are under the mouse while the left button is down. There is no problem
if I simply add 5 pixels to the size of the first column (or to LISTBROWSER_Left) but that might not be too robust.