Shortest Pangrammatic Window A pangram is a sentence or excerpt which contains all twenty-six letters of the alphabet, as is demonstrated in this code golf challenge. However, a pangrammatic window is a pangram in the form of some segment of text, which may end or begin halfway through a word, found somewhere within a larger work. These naturally occur everywhere, being proper subsets of true pangrams, so just verifying if something contains a pangrammatic window would be boring and also it was previously done.

So, we're interested in finding the smallest one there is in a given piece of text based on its letter length! In the shortest possible code in bytes, of course, to fit the theme.

Rules and Guidelines


Receive a string as the input and return  the string of the smallest pangrammatic window in the input if there is one. If there is not, return either a Boolean False or an empty string.
Whether a string is a pangrammatic window or not is case-insensitive and only depends on the 26 letters, not any punctuation or numbers or other odd symbols.
Similarly, a pangrammatic window's letter length is the total number of how many appearances of letters occur in it alone, and not simply the number of every character. The returned value must be smallest based on this count. We're linguists, after all, not programmers.
An output of a pangrammatic window must, however, must be an exact substring of the input, containing the same capitalization and punctuation, etc.
If there are multiple shortest pangrammatic windows of the same letter length, return any one of them.


Test Cases

'This isn't a pangram.'
==&gt; False

'Everyone knows about that infamous Quick-Brown-Fox (the one who jumped over some lazy ignoramus of a dog so many years ago).'
==&gt; 'Quick-Brown-Fox (the one who jumped over some lazy ig'

'"The five boxing wizards jump quickly." stated Johnny, before beginning to recite the alphabet with a bunch of semicolons in the middle. "ABCDEFGHI;;;;;;;;;;;;;;;JKLMNOPQRSTUVWXYZ!" he shouted to the heavens.'
==&gt; 'ABCDEFGHI;;;;;;;;;;;;;;;JKLMNOPQRSTUVWXYZ'
Pyth, 20 16 14 bytes
hol@GNf!-GrT0.:

Explanation:
             .: - substrings of input()
      f!-GrT0   - filter to ones which contain the alphabet
 ol@GN          - sort by number of alphabetical chars
h               - ^[0]

      f!-GrT0   - filter(lambda T:V, substrings)
          rT0   -    T.lower()
        -G      -   alphabet-^
       !        -  not ^

 o              - sort(^, lambda N:V)
   @GN          -   filter_presence(alphabet, N)
  l             -  len(^)

Try it here!
When there isn't a correct solution, the program exits with an error with no output to stdout.