ok I've found the problem
enter in your showthread_quickreply template(that you added to make the hack)..
Ok find this
Regular: <table cellpadding="4" cellspacing="1" border="0" width="100%">
The width is currently at 100, change 100 to your desired needs, in this case 95
I edited it for u
Edited: <table cellpadding="4" cellspacing="1" border="0" width="95%">
that should work, if not return to me