K (ngn/k), 15 bytes

{x@.=+|\2\!-#x}

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x:"abcdefg"
!-#x          / integers from -length-1 to -1
  -7 -6 -5 -4 -3 -2 -1
2\!-#x        / convert to base 2
  -1 -1 -1 -1 -1 -1 -1
   0  0  0  1  1  1  1
   0  1  1  0  0  1  1
   1  0  1  0  1  0  1
|\2\!-#x      / maximum scan
  -1 -1 -1 -1 -1 -1 -1
   0  0  0  1  1  1  1
   0  1  1  1  1  1  1
   1  1  1  1  1  1  1
.=+|\2\!-#x   / group identical columns
   (,0; 1 2; 3 4 5 6)
x@.=+|\2\!-#x / index back into the input string
   (,"a"; "bc"; "defg")