And what about pasting Python (or other offside languages)?
Are we pasting: 1. _at_ the levels indicated by the clipboard indentation, or 2. _at_ the level at the cursor, or 3. _into_ the level at the cursor?
So just taking deltas into account is still not enough because of 2. vs 3.
Too many choices, but the current behaviour of re-indenting the first line only is likely to be wrong most of the time and probably should be changed for multi-line pastes.