(vmotion): Use minibuf_prompt_width despite window-start.

src/indent.c

@@ -760,8 +760,11 @@ vmotion (from, vtarget, width, hscroll, window)
     = XTYPE (current_buffer->selective_display) == Lisp_Int
       ? XINT (current_buffer->selective_display)
 	: !NILP (current_buffer->selective_display) ? -1 : 0;
-  int start_hpos = (EQ (window, minibuf_window) && XWINDOW (window)->start == 1
-		    ? minibuf_prompt_width : 0);
+  /* The omission of the clause
+         && marker_position (XWINDOW (window)->start) == BEG
+     here is deliberate; I think we want to measure from the prompt
+     position even if the minibuffer window has scrolled.  */
+  int start_hpos = (EQ (window, minibuf_window) ? minibuf_prompt_width : 0);
 
  retry:
   if (vtarget > vpos)