Pressing tab on a blank line should match previous indentation level.

Related to the "move to start of line" issue, if the current indentation level is less than the previous line's, but the line is otherwise empty (nothing but whitespace), the whitespace should be expanded to the same indentation level as the last line and the cursor moved to end of line.

I'm so specific about the mechanics because the above implementation would avoid the "there's a tab on the line, but your cursor is at the beginning and you press tab again" problem (which in most implementations would result in the cursor at the correct indentation level with an extra tab after it.)