Noticed immediately with 6.4.5.2 install on Linux Mint (not tested on Winblows). Confirmed by another user on forum. 1) On beginning of an empty line, I am forbidden from entering a TAB character. NOTHING happens when TAB key is pressed. TAB is allowed if characters are present on the line, and are typed FIRST before the position on which tab is to be inserted. It then works fine if it is not the first item on the line. 2) Opening an existing document into which I inserted TAB characters in middle of lines, to force a certain formatting, with TABS being in middle of a sentence to manually "indent" it from the left side, has caused those pre-existing tab stops to be WIPED OUT. All formatting is now NOT how I intended to be. I want the old behavior back. If I want to put a tab character ANYWHERE I damn please, then I better be able to put a tab stop anywhere I want.
Looking at this a bit further, it appears that tab space is entered, it is just "useless" and "invisible". Tab spaces appear to be invisible because while they have stopped indenting space, however it takes same number of backspace hits as the number of tabs I have entered, before moving cursor position backwards, thus tab has become "invisible" and useless.
A known bug. Slipped into 6.4 branch with 6.4.5. Will be solved with 6.4.6. Alternative is reverting to 6.4.4 *** This bug has been marked as a duplicate of bug 134548 ***