x11: Fix window enumeration order when WM doesn't set |_NET_CLIENT_LI…

…ST_STACKING|

Chrome needs to know what window to send Xdnd events to during a
drag-and-drop operation.

It does so through |X11TopmostWindowFinder::FindWindowAt|, which calls
into |EnumerateWindows| when the WM has not set
|_NET_CLIENT_LIST_STACKING|. GNOME/mutter set this property, so this is
a little-tested code path.

However, setting this property is not mandated by the spec, and in
practice wlroots/Sway do not currently set it.

Chrome's current |EnumerateWindows| fallback path is incorrect,
resulting in downstream bugs like
swaywm/sway#5692 and
swaywm/wlroots#2889.

|EnumerateWindows| ends up calling |EnumerateChildren|, which uses
|XQueryTree| to determine the stacking order. |XQueryTree| returns
windows in bottom-to-top order, so the list has to be reverse-iterated
to get a top-down order that |FindWindowAt| expects (and to match the
order reported by |_NET_CLIENT_LIST_STACKING|).

The original code introduced in da11eed did so; however, it regressed in
3c64537 -- currently, the code comments are inconsistent with the actual
logic.

This commit switches |EnumerateChildren| to use a reverse iterator. It
is not used anywhere but as a fallback when |_NET_CLIENT_LIST_STACKING|
is not present, so this should be a safe change.

I have not touched the iteration order of Chrome's X11 menus. I suspect
that these are in the right order already.

TEST=manually tested on Sway 1.6, with Chromium overlapping an X11 gedit
     window

Change-Id: I8f777aa566db1e8d0614187fa4b3d156caa1e0f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2844104
Reviewed-by: Maksim Sisov <msisov@igalia.com>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#878767}

AUTHORS

@@ -1090,6 +1090,7 @@ Trent Willis <trentmwillis@gmail.com>
 Trevor Perrin <unsafe@trevp.net>
 Tripta Gupta <tripta.g@samsung.com>
 Tristan Fraipont <tristan.fraipont@gmail.com>
+Tudor Brindus <me@tbrindus.ca>
 Tuukka Toivonen <tuukka.toivonen@intel.com>
 U. Artie Eoff <ullysses.a.eoff@intel.com>
 Umar Hansa <umar.hansa@gmail.com>

ui/base/x/x11_util.cc

@@ -719,10 +719,10 @@ bool EnumerateChildren(EnumerateWindowsDelegate* delegate,
     return false;
 
   std::vector<x11::Window> windows;
-  std::vector<x11::Window>::iterator iter;
   if (depth == 0) {
     XMenuList::GetInstance()->InsertMenuWindows(&windows);
     // Enumerate the menus first.
+    std::vector<x11::Window>::iterator iter;
     for (iter = windows.begin(); iter != windows.end(); iter++) {
       if (delegate->ShouldStopIterating(*iter))
         return true;
@@ -737,7 +737,8 @@ bool EnumerateChildren(EnumerateWindowsDelegate* delegate,
 
   // XQueryTree returns the children of |window| in bottom-to-top order, so
   // reverse-iterate the list to check the windows from top-to-bottom.
-  for (iter = windows.begin(); iter != windows.end(); iter++) {
+  std::vector<x11::Window>::reverse_iterator iter;
+  for (iter = windows.rbegin(); iter != windows.rend(); iter++) {
     if (IsWindowNamed(*iter) && delegate->ShouldStopIterating(*iter))
       return true;
   }
@@ -747,7 +748,7 @@ bool EnumerateChildren(EnumerateWindowsDelegate* delegate,
   // loop because the recursion and call to XQueryTree are expensive and is only
   // needed for a small number of cases.
   if (++depth <= max_depth) {
-    for (iter = windows.begin(); iter != windows.end(); iter++) {
+    for (iter = windows.rbegin(); iter != windows.rend(); iter++) {
       if (EnumerateChildren(delegate, *iter, max_depth, depth))
         return true;
     }