Strange behavior of cmd.exe when hammered with clear screen operations from Cygwin program.

[Kaz Kylheku]

On 26.07.2016 22:19, Kaz Kylheku wrote:
> The test program featured below works fine under
> the Cygwin console window.
> 
> If executed from a cmd.exe console, strange things happen with the
> cmd.exe window. Suddenly, the window spontaneously resizes so that
> is taller than the desktop. A horizontal scrollbar may appear in it
> also, as well as an unpainted area through which the desktop is
> visible. As soon as you try to grab the title bar to resize it, it
> disappears. It hasn't quit; it is actually minimized. You can
> maximize it from the task bar and then resize the tall window to
> its original shape.

Narrowed it down to the con.scroll_window call in
fhandler_console::clear_screen.

The following test change makes the behavior go away. Moreover,
the repeated clear screen requests stop making the vertical scroll
bar jump around like crazy:

diff --git a/winsup/cygwin/fhandler_console.cc 
b/winsup/cygwin/fhandler_console.cc
index 5fe4480..2b8acb5 100644
--- a/winsup/cygwin/fhandler_console.cc
+++ b/winsup/cygwin/fhandler_console.cc
@@ -1261,7 +1261,9 @@ fhandler_console::clear_screen (cltype xc1, cltype 
yc1, cltype xc2, cltype yc2)

    /* Detect special case - scroll the screen if we have a buffer in 
order to
       preserve the buffer. */
+#if 0
    if (!con.scroll_window (h, x1, y1, x2, y2))
+#endif
      con.clear_screen (h, x1, y1, x2, y2);
  }


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