Revert unrelated change "Remove irrelevant self.dch1 comment and unused px_pos variable"

This reverts commit 742096d.

Author: tanloong

Lib/_pyrepl/windows_console.py

@@ -249,10 +249,22 @@ def input_hook(self):
     def __write_changed_line(
         self, y: int, oldline: str, newline: str, px_coord: int
     ) -> None:
+        # this is frustrating; there's no reason to test (say)
+        # self.dch1 inside the loop -- but alternative ways of
+        # structuring this function are equally painful (I'm trying to
+        # avoid writing code generators these days...)
         minlen = min(wlen(oldline), wlen(newline))
         x_pos = 0
         x_coord = 0
 
+        px_pos = 0
+        j = 0
+        for c in oldline:
+            if j >= px_coord:
+                break
+            j += wlen(c)
+            px_pos += 1
+
         # reuse the oldline as much as possible, but stop as soon as we
         # encounter an ESCAPE, because it might be the start of an escape
         # sequence