The handler responsible for converting tab to spaces replaces the selection instead of indenting it:

I would also expect ShiftTab to unindent a line or selection, but it doesn't have any effect.

Observed behavior:

Expected behavior (shown on sublime text):

Workaround:

Using multiple cursors by pressing Alt and dragging the mouse can indent multiple lines, but it isn't practical and also fails at unindenting correctly as it removes the whole indentation when pressing ShiftTab.