fixes #19846; std/unicode.strip trailing big chars (#25274)

fixes #19846

lib/pure/unicode.nim

@@ -1037,6 +1037,19 @@ proc split*(s: openArray[char], sep: Rune, maxsplit: int = -1): seq[string] {.no
   ## that returns a sequence of substrings.
   accResult(split(s, sep, maxsplit))
 
+func getRuneHeadIdx(s: openArray[char], idx: int): int =
+  ## Given `[idx]` is within a Rune, then `s[result]` is the first byte of that Rune.
+  result = idx
+  if s[result] <= '\x7F': # 0b0111_1111
+    return
+  # 0b1...
+  dec result
+  for _ in 0..1:
+    if s[result] >= '\xC0': # 0b11xx_xxxx
+      # 0b110... or 0b1110...
+      return
+    dec result
+
 proc strip*(s: openArray[char], leading = true, trailing = true,
             runes: openArray[Rune] = unicodeSpaces): string {.noSideEffect,
             rtl, extern: "nucStrip".} =
@@ -1073,18 +1086,9 @@ proc strip*(s: openArray[char], leading = true, trailing = true,
       xI: int
       rune: Rune
     while i >= 0:
+      i = getRuneHeadIdx(s, i)
       xI = i
       fastRuneAt(s, xI, rune)
-      var yI = i - 1
-      while yI >= 0:
-        var
-          yIend = yI
-          pRune: Rune
-        fastRuneAt(s, yIend, pRune)
-        if yIend < xI: break
-        i = yI
-        rune = pRune
-        dec(yI)
       if not runes.contains(rune):
         eI = xI - 1
         break