Can you be a little more detailed in your description? So far I cannot reproduce this problem. Working in C++ I started with this code:
static void testGeneralCode()
{
int nFoo;
}
Then I added a comment after the variable, giving me:
static void testGeneralCode()
{
int nFoo; // comment here
}
then using the keyboard I selected the entire line (I tried both Shift + Up arrow and Shift + Down arrow) and then pressed / to comment out the line. The line is always commented out correctly for me.