This bug started in Slack 3.4.0 and affects Xubuntu 18.04.2 LTS too.
The problem happens because xfwm4 looks for any of the following:
"_NET_WM_WINDOW_TYPE_DESKTOP",
"_NET_WM_WINDOW_TYPE_DIALOG",
"_NET_WM_WINDOW_TYPE_DOCK",
"_NET_WM_WINDOW_TYPE_MENU",
"_NET_WM_WINDOW_TYPE_NORMAL",
"_NET_WM_WINDOW_TYPE_SPLASH",
"_NET_WM_WINDOW_TYPE_TOOLBAR",
"_NET_WM_WINDOW_TYPE_UTILITY",
But Slack doesn't set them.
I compiled xfwm4 from source, debugged remotely (it is nearly impossible
to halt the window manager in the host machine...) and the problem boils
down to:
1. clientCycleCreateList in cycle.c calls:
if (!clientSelectMask (c2, NULL, search_range, WINDOW_REGULAR_FOCUSABLE))
{
TRACE ("%s not in select mask", c2->name);
continue;
}
2. Inside clientSelectMask in focus.c, c2->type = 0; thus "return
c2->type & mask" returns 0 regardless of the mask and thinks the window
should not be alt-tabbed
3. I believe (not 100% sure) this happens because in clientWindowType in
netwm.c, c->type_atom == None. I don't know whether this should be
treated as a client bug or a window manager bug. I'm not an X11 expert
so I don't know if it is possible to know if a window is the root one,
but root windows should definitely be alt-tabbeable.
If I change the line to:
if (!clientSelectMask (c2, NULL, search_range,
WINDOW_REGULAR_FOCUSABLE) && c2->type != UNSET)
Then Slack works with alt+tab and my computer hasn't blown up yet. I
don't know if it's the proper fix though, but it may be
I fixed this problem by rebuilding xfwm4 with the attached patch:
mkdir tmp
cd tmp
sudo apt install devscripts build-essential fakeroot dpkg-dev
sudo apt-get build-dep xfwm4
apt-get source xfwm4
cd xfwm4-4.12.5
patch -i xfwm4_slack_fix.diff #Point it where the patch is located!!!
debuild -b -uc -us
sudo dpkg -i ../xfwm4_4.12.5-1ubuntu0.18.04.1_amd64.deb
** Patch added: "Alt + Tab fix for xfwm4 and Slack"
https://bugs.launchpad.net/ubuntu/+source/xfwm4/+bug/1827302/+attachment/5261320/+files/xfwm4_slack_fix.diff
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https://bugs.launchpad.net/bugs/1827302
Title:
18.10 → 19.04 Alt-Tab cycling no longer includes Slack window
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