Cygwin: pty: Use redraw screen instead of clear screen.

- Previously, pty cleared screen at startup for synchronization
  between the real screen and console screen buffer for pseudo
  console. With this patch, instead of clearing screen, the screen
  is redrawn when the first native program is executed after pty
  is created. In other words, synchronization is deferred until
  the native app is executed. Moreover, this realizes excluding
  $TERM dependent code.

winsup/cygwin/fhandler_tty.cc

@@ -2669,7 +2669,7 @@ fhandler_pty_slave::fixup_after_attach (bool native_maybe, int fd_set)
 	  if (get_ttyp ()->num_pcon_attached_slaves == 0
 	      && !ALWAYS_USE_PCON)
 	    /* Assume this is the first process using this pty slave. */
-	    get_ttyp ()->need_clear_screen = true;
+	    get_ttyp ()->need_redraw_screen = true;
 
 	  get_ttyp ()->num_pcon_attached_slaves ++;
 	}
@@ -2700,6 +2700,21 @@ fhandler_pty_slave::fixup_after_attach (bool native_maybe, int fd_set)
 		  kill (get_ttyp ()->pcon_pid, 0) != 0)
 		get_ttyp ()->pcon_pid = myself->pid;
 	      get_ttyp ()->switch_to_pcon_out = true;
+
+	      if (get_ttyp ()->need_redraw_screen)
+		{
+		  /* Forcibly redraw screen based on console screen buffer. */
+		  /* The following code triggers redrawing the screen. */
+		  CONSOLE_SCREEN_BUFFER_INFO sbi;
+		  GetConsoleScreenBufferInfo (get_output_handle (), &sbi);
+		  SMALL_RECT rect = {0, 0,
+		    (SHORT) (sbi.dwSize.X -1), (SHORT) (sbi.dwSize.Y - 1)};
+		  COORD dest = {0, 0};
+		  CHAR_INFO fill = {' ', 0};
+		  ScrollConsoleScreenBuffer (get_output_handle (),
+					     &rect, NULL, dest, &fill);
+		  get_ttyp ()->need_redraw_screen = false;
+		}
 	    }
 	  init_console_handler (false);
 	}
@@ -2717,19 +2732,6 @@ fhandler_pty_slave::fixup_after_fork (HANDLE parent)
   // fork_fixup (parent, inuse, "inuse");
   // fhandler_pty_common::fixup_after_fork (parent);
   report_tty_counts (this, "inherited", "");
-
-  if (get_ttyp ()->need_clear_screen)
-    {
-      const char *term = getenv ("TERM");
-      if (term && strcmp (term, "dumb") && !strstr (term, "emacs"))
-	{
-	  /* FIXME: Clearing sequence may not be "^[[H^[[J"
-	     depending on the terminal type. */
-	  DWORD n;
-	  WriteFile (get_output_handle_cyg (), "\033[H\033[J", 6, &n, NULL);
-	}
-      get_ttyp ()->need_clear_screen = false;
-    }
 }
 
 void

winsup/cygwin/tty.cc

@@ -244,7 +244,7 @@ tty::init ()
   pcon_pid = 0;
   num_pcon_attached_slaves = 0;
   term_code_page = 0;
-  need_clear_screen = false;
+  need_redraw_screen = false;
 }
 
 HANDLE

winsup/cygwin/tty.h

@@ -105,7 +105,7 @@ private:
   pid_t pcon_pid;
   int num_pcon_attached_slaves;
   UINT term_code_page;
-  bool need_clear_screen;
+  bool need_redraw_screen;
 
 public:
   HANDLE from_master () const { return _from_master; }