Fix removal of frame decorations on Windows (Bug#38705)

* src/w32fns.c (w32_set_undecorated): Actualize
f->output_data.w32->dwStyle for subsequent calls of
AdjustWindowRect (Bug#38705).
* src/w32term.h (struct w32_output): Add comment for
dwstyle slot.

2 files changed, 7 insertions, 0 deletions

diff --git a/src/w32fns.c b/src/w32fns.c
index 75e0d531a23..61e22e57009 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -2146,6 +2146,9 @@ w32_set_undecorated (struct frame *f, Lisp_Object new_value, Lisp_Object old_val
                     | SWP_FRAMECHANGED);
       FRAME_UNDECORATED (f) = false;
     }
+
+  f->output_data.w32->dwStyle = GetWindowLong (hwnd, GWL_STYLE);
+
   unblock_input ();
 }
 
diff --git a/src/w32term.h b/src/w32term.h
index 5a54f542365..737764b8942 100644
--- a/src/w32term.h
+++ b/src/w32term.h
@@ -371,6 +371,10 @@ struct w32_output
   /* Non-hourglass cursor that is currently active.  */
   HCURSOR current_cursor;
 
+  /* The window style for this frame.  Set up when the frame is
+     created and updated when adding/removing decorations in
+     w32_set_undecorated.  Used by w32_set_window_size to adjust the
+     frame's window rectangle.  */
   DWORD dwStyle;
 
   /* This is the Emacs structure for the display this frame is on.  */