Re: Bug in CVS Emacs frame positioning under X

["Fran Litterio" <flitterio@gmail.com>]

On 3/29/06, Richard Stallman wrote:
> It could be that we could solve this by completely reorganizing the
> way that code works.  The person who wrote it was not the best
> programmer, and I then tried to fix it up with just a partial
> understanding of how to use Xt.

I can't claim to be an expert at X programming, but I've changed the
frame positioning algorithm in src/xterm.c so that it copes with the
non-deterministic nature of X event timing, and so that it fixes the
frame positioning bug I reported earlier in this thread.  Patches to
src/xterm.h and src/xterm.c are below.

Here's what I did:

First, I tried Jan Djärv's suggestion to call XSync() to force Xlib to
wait for the X server to handle the XMoveWindow() request, but that
failed to make XGetGeometry() see up-to-date position information for
the just-moved frame. Instead, I wrote a new function,
x_sync_with_move() that uses XSync() in a loop to make sure
XGetGeometry() sees up-to-date position information after a frame is
moved.

Frankly, this is not an elegant solution -- it is periliously close to
busy-waiting (except that XSync() blocks). But my testing has shown
that it guarantees that we know the up-to-date position of
recently-moved frames, which is a requirement for detecting Type A
window managers (i.e., WMs that misposition frames by the width and
height of the WM decorations). Also, the loop in x_sync_with_move() is
bounded so that it cannot excessively consume cycles.

Then I changed x_set_offset() to perform these calls in this order:

   XMoveWindow()
   x_sync_with_move()
   x_check_expected_move()

I rewrote x_check_expected_move() so that it calls XMoveWindow()
directly to compensate for Type A window managers and the top/left
adjustments are recorded for future use with that frame. The
adjustments are stored in struct x_output, so the behavior is correct
even if the user's window manager draws differently sized decorations
around different Emacs frames (e.g., FVWM can be configured to do
this).

I removed the call to x_check_expected_move() from the code that
handles ConfigureNotify events, because it is now called synchronously
immediately after XMoveWindow().

I am currently running with this patch installed, and I'm seeing
completely deterministic frame positioning that accurately
compenstates for my Type A window manager (FVWM). I will test it
against twm and mwm.  If others can test it against their favorite WM,
I would greatly appreciate it.

A good acid test is to evaluate the following Elisp:

        (dolist (i '(1 2 3 4 5 6 7 8 9 a b c d e))
         (let ((frame (make-frame '((top . 50) (left . 50)))))
           (set-frame-position frame 200 200)
           (set-frame-position frame 300 300)))

All of the frames should end up exactly at x/y position 300/300.
--
Francis Litterio
flitterio@gmail.com
http://world.std.com/~franl/
GPG and PGP public keys available on keyservers.


Index: xterm.h
===================================================================
RCS file: /cvsroot/emacs/emacs/src/xterm.h,v
retrieving revision 1.183
diff -w -u -r1.183 xterm.h
--- xterm.h	18 Mar 2006 13:49:11 -0000	1.183
+++ xterm.h	1 Apr 2006 13:56:52 -0000
@@ -637,18 +637,14 @@
      FocusOut and LeaveNotify clears EXPLICIT/IMPLICIT. */
   int focus_state;

-  /* The latest move we made to FRAME_OUTER_WINDOW.  Saved so we can
-     compensate for type A WMs (see wm_type in dpyinfo above).  */
-  int expected_top;
-  int expected_left;
-
   /* The offset we need to add to compensate for type A WMs.  */
   int move_offset_top;
   int move_offset_left;

-  /* Nonzero if we have made a move and needs to check if the WM placed us
-     at the right position.  */
-  int check_expected_move;
+  /* The frame's left/top offsets before we call XMoveWindow().  See
+     x_check_expected_move(). */
+  int left_before_move;
+  int top_before_move;
 };

 #define No_Cursor (None)


Index: xterm.c
===================================================================
RCS file: /cvsroot/emacs/emacs/src/xterm.c,v
retrieving revision 1.909
diff -w -u -r1.909 xterm.c
--- xterm.c	24 Mar 2006 15:24:20 -0000	1.909
+++ xterm.c	1 Apr 2006 13:58:21 -0000
@@ -366,7 +366,8 @@
 					    Lisp_Object *, Lisp_Object *,
 					    unsigned long *));
 static void x_check_fullscreen P_ ((struct frame *));
-static void x_check_expected_move P_ ((struct frame *));
+static void x_check_expected_move P_ ((struct frame *, int, int));
+static void x_sync_with_move(struct frame *, int, int, int);
 static int handle_one_xevent P_ ((struct x_display_info *, XEvent *,
 				  int *, struct input_event *));

@@ -6661,11 +6662,8 @@
               && GTK_WIDGET_MAPPED (FRAME_GTK_OUTER_WIDGET (f)))
 #endif
             {
-	      /* What we have now is the position of Emacs's own window.
-		 Convert that to the position of the window manager window.  */
 	      x_real_positions (f, &f->left_pos, &f->top_pos);

-	      x_check_expected_move (f);
 	      if (f->want_fullscreen & FULLSCREEN_WAIT)
 		f->want_fullscreen &= ~(FULLSCREEN_WAIT|FULLSCREEN_BOTH);
             }
@@ -8212,8 +8210,11 @@
 {
   int modified_top, modified_left;

-  if (change_gravity > 0)
+  if (change_gravity != 0)
     {
+      FRAME_X_OUTPUT (f)->left_before_move = f->left_pos;
+      FRAME_X_OUTPUT (f)->top_before_move = f->top_pos;
+
       f->top_pos = yoff;
       f->left_pos = xoff;
       f->size_hint_flags &= ~ (XNegative | YNegative);
@@ -8231,7 +8232,7 @@
   modified_left = f->left_pos;
   modified_top = f->top_pos;

-  if (FRAME_X_DISPLAY_INFO (f)->wm_type == X_WMTYPE_A)
+  if (change_gravity != 0 && FRAME_X_DISPLAY_INFO (f)->wm_type == X_WMTYPE_A)
     {
       /* Some WMs (twm, wmaker at least) has an offset that is smaller
          than the WM decorations.  So we use the calculated offset instead
@@ -8243,12 +8244,26 @@
   XMoveWindow (FRAME_X_DISPLAY (f), FRAME_OUTER_WINDOW (f),
                modified_left, modified_top);

-  if (FRAME_VISIBLE_P (f)
-      && FRAME_X_DISPLAY_INFO (f)->wm_type == X_WMTYPE_UNKNOWN)
+  x_sync_with_move(f, f->left_pos, f->top_pos,
+		   FRAME_X_DISPLAY_INFO (f)->wm_type == X_WMTYPE_UNKNOWN ? 1 : 0);
+
+  /* change_gravity is non-zero when this function is called from Lisp to
+     programmatically move a frame.  In that case, we call
+     x_check_expected_move() to discover if we have a "Type A" or "Type B"
+     window manager, and, for a "Type A" window manager, adjust the position
+     of the frame.
+
+     We call x_check_expected_move() if a programmatic move occurred, and
+     either the window manager type (A/B) is unknown or it is Type A but we
+     need to compute the top/left offset adjustment for this frame.
+     */
+
+  if (change_gravity != 0 &&
+      (FRAME_X_DISPLAY_INFO (f)->wm_type == X_WMTYPE_UNKNOWN ||
+       (FRAME_X_DISPLAY_INFO (f)->wm_type == X_WMTYPE_A &&
+	(FRAME_X_OUTPUT (f)->move_offset_left == 0 && FRAME_X_OUTPUT
(f)->move_offset_top == 0))))
     {
-      FRAME_X_OUTPUT (f)->check_expected_move = 1;
-      FRAME_X_OUTPUT (f)->expected_top = f->top_pos;
-      FRAME_X_OUTPUT (f)->expected_left = f->left_pos;
+    x_check_expected_move(f, modified_left, modified_top);
     }

   UNBLOCK_INPUT;
@@ -8284,37 +8299,93 @@
     }
 }

-/* If frame parameters are set after the frame is mapped, we need to move
-   the window.
-   Some window managers moves the window to the right position, some
-   moves the outer window manager window to the specified position.
-   Here we check that we are in the right spot.  If not, make a second
-   move, assuming we are dealing with the second kind of window manager. */
+/* This function is called by x_set_offset() to determine whether the window
+   manager interfered with the positioning of the frame.  Type A window
+   managers position the surrounding window manager decorations a small
+   amount above and left of the user-supplied position.  Type B window
+   managers position the surrounding window manager decorations at the
+   user-specified position.  If we detect a Type A window manager, we
+   compensate by moving the window right and down by the proper amount. */
+
 static void
-x_check_expected_move (f)
+x_check_expected_move (f, expected_left, expected_top)
      struct frame *f;
+     int expected_left;
+     int expected_top;
 {
-  if (FRAME_X_OUTPUT (f)->check_expected_move)
-  {
-    int expect_top = FRAME_X_OUTPUT (f)->expected_top;
-    int expect_left = FRAME_X_OUTPUT (f)->expected_left;
+  int count = 0, current_left = 0, current_top = 0;
+
+  /* x_real_positions() returns the left and top offsets of the outermost
+     window manager window around the frame. */
+
+  x_real_positions (f, &current_left, &current_top);

-    if (expect_top != f->top_pos || expect_left != f->left_pos)
+  if (current_left != expected_left || current_top != expected_top)
       {
+    /* It's a "Type A" window manager. */
+
         FRAME_X_DISPLAY_INFO (f)->wm_type = X_WMTYPE_A;
-        FRAME_X_OUTPUT (f)->move_offset_left = expect_left - f->left_pos;
-        FRAME_X_OUTPUT (f)->move_offset_top = expect_top - f->top_pos;
+    FRAME_X_OUTPUT (f)->move_offset_left = expected_left - current_left;
+    FRAME_X_OUTPUT (f)->move_offset_top = expected_top - current_top;
+
+    /* Now fix the mispositioned frame's location. */

-        f->left_pos = expect_left;
-        f->top_pos = expect_top;
-        x_set_offset (f, expect_left, expect_top, 0);
+    int adjusted_left = expected_left + FRAME_X_OUTPUT (f)->move_offset_left;
+    int adjusted_top = expected_top + FRAME_X_OUTPUT (f)->move_offset_top;
+
+    XMoveWindow (FRAME_X_DISPLAY (f), FRAME_OUTER_WINDOW (f),
+		 adjusted_left, adjusted_top);
+
+    x_sync_with_move(f, expected_left, expected_top, 0);
       }
-    else if (FRAME_X_DISPLAY_INFO (f)->wm_type == X_WMTYPE_UNKNOWN)
+  else
+  {
+    /* It's a "Type B" window manager.  We don't have to adjust the
+       frame's position. */
+
       FRAME_X_DISPLAY_INFO (f)->wm_type = X_WMTYPE_B;
+  }
+}
+

-    /* Just do this once */
-    FRAME_X_OUTPUT (f)->check_expected_move = 0;
+/* Wait for XGetGeometry() to return up-to-date position information for a
+   recently-moved frame.  Call this immediately after calling XMoveWindow().
+   If FUZZY is non-zero, then LEFT and TOP are just estimates of where the
+   frame has been moved to, so we use a fuzzy position comparison instead
+   of an exact comparison. */
+
+static void
+x_sync_with_move(f, left, top, fuzzy)
+    struct frame * f;
+    int left, top, fuzzy;
+{
+  int count = 0;
+
+  while (count++ < 50)
+  {
+    int current_left = 0, current_top = 0;
+
+    /* In theory, this call to XSync() only needs to happen once, but
in practice,
+       it doesn't seem to work, hence the need for the surrounding loop. */
+
+    XSync(FRAME_X_DISPLAY (f), False);
+    x_real_positions (f, &current_left, &current_top);
+
+    if (fuzzy)
+    {
+      /* The left fuzz-factor is 10 pixels.  The top fuzz-factor is
40 pixels. */
+
+      if (abs(current_left - left) <= 10 && abs(current_top - top) <= 40)
+	return;
   }
+    else if (current_left == left && current_top == top)
+      return;
+  }
+
+  /* As a last resort, just wait 0.5 seconds and hope that XGetGeometry()
+     will then return up-to-date position info. */
+
+  wait_reading_process_output(0, 500000, 0, 0, Qnil, NULL, 0);
 }