Dear Maintainer,

*** Reporter, please consider answering these questions, where appropriate ***

Alt-Tab usually cycles through apps chronologically, but after a few hours of
uptime, the selection order becomes random.

I've found that disabling 'Threads' (Hyper-Threading) in the BIOS resolves this
bug permanently.

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Hi Frederic,

thanks for the report, and sorry for the long delay.

Some background on how Alt-Tab works in Cinnamon, because it explains what
could be going on. Unlike most window managers, the Cinnamon app
switcher does
not use the window manager's own MRU (most recently used) list. It takes the
list of windows in stacking order and re-sorts it by each window's
_NET_WM_USER_TIME property (js/ui/appSwitcher/appSwitcher.js). That
property is
only partly under the window manager's control: applications set it
themselves,
and a client sending a bad timestamp can end up with a window that stays
pinned
at the top of the order for the rest of the session. That would match
"the order
becomes random after a few hours of uptime".

Hyper-Threading, on the other hand, is unlikely to be the real variable
here:
the ordering is a plain single-threaded computation in Cinnamon, and it
does not
depend on the number of logical CPUs. What changing that BIOS setting does
require is a reboot, and a reboot is what clears the accumulated timestamps.

Do you still see this, and would you be willing to help debug it? If so,
these
three things would tell us what is going on:

1. Does the problem also go away if you just restart the session (log
out and
    log back in), or restart Cinnamon (Ctrl+Alt+Escape), without
touching the
    BIOS setting? This is the key test to tell a reboot apart from Hyper-
    Threading.

2. The output of:

        grep "comparison timestamp" ~/.xsession-errors

    Lines like "last_user_time (...) is greater than comparison
timestamp (...)"
    would confirm that some application is sending bad timestamps.

3. While the problem is happening, please open Looking Glass (Alt+F2,
then type
    "lg"), go to the "Evaluator" tab and run these two lines, then send
us both
    outputs:

        global.get_window_actors().map(a => a.get_meta_window()).map(w
=> w.get_wm_class() + " | " + w.get_title() + " | " + w.get_user_time()
+ " | min=" + w.minimized).join("\n")

        global.display.get_tab_list(imports.gi.Meta.TabList.NORMAL,
global.workspace_manager.get_active_workspace()).map(w =>
w.get_wm_class() + " | " + w.get_title()).join("\n")

    The first one is the order Alt-Tab uses, the second one is the window
    manager's real MRU order. If the second is correct and the first is
not, the
    diagnosis above is confirmed.

One more thing worth knowing: there is no upstream report about this.
The few
related upstream issues ask for the opposite behaviour, that is, a static
Alt-Tab order instead of the most-recently-used one ([1], [2], the
latter closed
by a stale bot without a resolution). So a precise, reproducible description
from you is what it would take to get this looked at upstream.

Thanks,

[1] https://github.com/linuxmint/cinnamon/issues/8758
[2] https://github.com/linuxmint/cinnamon/issues/9410