Some windows change size. "The longest stretch of text with no repeated character": grow the window to the right; when a repeat appears, shrink it from the left until the repeat is gone.

def longest_unique(text):
    seen = set()
    left = 0
    best = 0
    for right, ch in enumerate(text):
        while ch in seen:
            seen.remove(text[left])
            left += 1
        seen.add(ch)
        best = max(best, right - left + 1)
    return best

print(longest_unique("abcabcbb"))
print(longest_unique("pwwkew"))

Each character enters and leaves the window at most once, so it's still O(n).