> 1. load a C file with an ifdef block
 > 2. turn on hide-ifdef-mode
 > 3. hide-ifdef-block and show-ifdef-block work fine
 > 4. set hide-ifdef-lines to t
 > 5. hide-ifdef-block works, but show-ifdef-block only brings back one line
 >    of this block each time it is called

Attached find two patches.  The first patches just `show-ifdef-block'.
The second uses one invisibility overlay only to cover the ifdef lines
and the enclosed block.  Without any warranty.