Re: Mouse pointer warping with gtk display

[Howard Spoelstra <hsp.cat7@gmail.com>]

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On Thu, Jan 27, 2022 at 8:48 PM Marc-André Lureau <
marcandre.lureau@gmail.com> wrote:

> Hi
>
> On Thu, Jan 27, 2022 at 10:29 PM BALATON Zoltan <balaton@eik.bme.hu>
> wrote:
>
>> Hello,
>>
>> We've found a problem with mouse pointer jumping around then constantly
>> warping making it unusable which only seems to happen with the gtk
>> display. Similar problem was reported with MacOS guests but now we could
>> also reproduce with MorphOS on pegasos2 and thus confirm it's not a guest
>> issue or hardware emulation problem but seems to be related to QEMU's gtk
>> display. This can be reproduced as following (boot.img is the file from
>> the root dir of the iso which is available from
>> https://www.morphos-team.net/morphos-3.15.iso ):
>>
>> qemu-system-ppc -M pegasos2 -cdrom morphos-3.15.iso -display sdl \
>> -L ../pc-bios -kernel boot.img -device ati-vga,romfile=""
>>
>> This does not have this problem and mouse works but running with -display
>> gtk has the mouse pointer issue making it unusable. It can be seemingly
>> avoided with using -device ati-vga,romfile="",guest_hwcursor=true so I
>> think problem is somewhere in mouse handling in ui/gtk.c with host side
>> mouse pointer. The gd_motion_event() in that file has a big comment about
>> warping mouse back to the middle near the end of the function which seems
>> to ne what's happening and causing this as this does not appear in sdl.c
>> that does not show the same issue but I don't understand what that code
>> is
>> trying to do.
>>
>
> In general, a guest handles mouse input with relative motion events
> (standard ps2).
>
> A more VM-friendly guest can additionally handle mouse input with absolute
> coordinates (vmmouse, usb tablet etc)
>
> While giving absolute coordinates to a guest is quite a simple task from a
> client (and offers some advantages) it is more tricky to deal with relative
> mouse events...
>
> At the toolkit level, iirc, you generally don't have low-level events &
> relative motion (gtk doesn't let you grab the mouse exclusively to receive
> all events). You can workaround this with some window managers (ex wayland
> relative-pointer unstable extension).
>
> The "old" strategy with gtk2/3 is to grab the mouse, which is then
> constrained within the display widget iirc, and convert absolute events to
> relative events by doing simple current_position - last_position. Since
> your client mouse may reach the widget borders (and/or the client monitor
> limits, can't remember which applies, I think it varies), you also need to
> recenter the client mouse somewhere, and that's usually in the middle of
> the display widget.
>
> This is clunky at best, and gtk devs decided to no longer support that
> kind of API with gtk4. The recommended approach is to use lower-level
> events, like the one you get from the wayland extensions. This is a pain
> for portability, but that's all we have for now.
>
> spice-gtk implements the wayland stuff. Have you tried running -display
> spice-app? It may have a more polished behavior than the one in qemu.
>
>
>
>
>
>
>>
>> It's a known problem that host mouse pointer can get out of sync with the
>> guest which I think is because mouse acceleration on the host is not
>> correctly taken into account when reporting mouse moves to the guest and
>> this may try to correct that issue but it seems to cause more harm than
>> good in some cases. Any idea what might be the problem and how to solve
>> this? I wonder why nobody else is seeing this with other guests. (I'm not
>> direcly interested in fixing it as I'm using sdl display but this makes
>> it
>> hard to use some guests for users whose distros make gtk backend the
>> default which may actually be most people so it might be a good idea to
>> improve this for them so I thought at least reporting the problem is
>> useful.)
>>
>
> I suspect SDL offers an API for relative mouse events, which works better
> than what we do in the gtk backend.
>
> I hope in the future we can drop the gtk code from qemu, in favor of
> dbus-display and rdw. Maintaining that kind of code is hard, and having the
> same gtk widget base code for various remote desktop apps & protocols
> should help maintenance and offer a consistent behavior, whether you use
> QEMU/DBus, VNC, Spice, or RDP and whatever the future brings.
>
> (I wrote the above mostly from memory, I agree it's convoluted and a bit
> buggy, I doubt anyone likes it ;)
>
> --
> Marc-André Lureau
>

Hi Zoltan,

While I can reproduce the issue you report when running morphos with gtk, I
cannot currently reproduce with the Mac OS/OSX guests I tested on Linux
host. Both mac99 (adb mouse and kbd) and mac99,via=pmu (usb mouse and kbd)
keep the mouse inside the guest window with both sdl and gtk.

Best,
Howard

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