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The current behavior of IDLE's feature is standard, not a bug. The cpython respository does not currently accept .py, .c, or .rst files with any trailing whitespace. <pythondir>/tools/scripts/patchcheck.py strips trailing whitespace from all lines. Notepad++, a widely used multi-language Windows programmer's editor, does the same.
I am aware of the potential surprise. If one wants multiline strings with embedded trailing whitespace, one should avoid trailing whitespace strippers or use Python's string concatenation feature. 
>>> s = ('space \n' 'tab\t\n' 'line3\n')
>>> s
'space \ntab\t\nline3\n'
I am revising the doc to make the rstring behavior clearer.