Select multiple lines, press Tab. I would expect a tab to be entered replacing the selected text.
Instead, tabs are added at the beginning of all lines.
Interesting, the same behaviour with Notepad++. Not though with MS Word, or LibreOffice Writer.
The point is that the first the last line are not wholly selected, but partly selected. The behaviour I would expect is the behaviour one gets now by pressing 1) DEL 2) TAB.
If you don’t like the current behavior, go to Properties for All Configurations (Tools menu), select Keyboard page, select Edit category, select Tab or Increase Line Indent command, select Tab form the Current Keys list, and click Delete button.