Re: [PATCH qemu] ui/sdl2: add grab-on-tablet option for absolute input devices

[Markus Armbruster <armbru@redhat.com>]

I apologize for the lateness of my review.

~rongyichang <rongyichang@git.sr.ht> writes:

> From: rongyichang <rongyichang@xiaomi.com>
>
> When using absolute coordinate input devices (e.g. virtio-tablet-device),
> the SDL display backend implements a "soft grab" where the mouse is
> automatically grabbed when it enters the window interior and ungrabbed
> when it hits the window edge. This edge-ungrab behavior causes problems
> in embedded emulation scenarios:
>
> 1. Mouse escapes the SDL window at edges
> 2. SDL does not deliver BUTTONUP events for the escaped mouse
> 3. The guest gets stuck in a pressed/touch-down state
> 4. First click back into the window is silently dropped
>
> This issue is confirmed as a known SDL limitation (SDL issue #5301).
>
> Add a new -display sdl,grab-on-tablet=on option that makes absolute
> coordinate devices use the same grab behavior as relative (mouse)
> devices: user must click to grab, Ctrl+Alt+G to release, and no
> automatic grab on window enter or edge-based ungrab/regrab.
>
> Signed-off-by: Yichang Rong <rongyichang@xiaomi.com>
> ---
>  qapi/ui.json    |  8 +++++++-
>  qemu-options.hx |  9 +++++++--
>  ui/sdl2.c       | 16 +++++++++++++---
>  3 files changed, 27 insertions(+), 6 deletions(-)
>
> diff --git a/qapi/ui.json b/qapi/ui.json
> index b2c42a7f57..454d9041bf 100644
> --- a/qapi/ui.json
> +++ b/qapi/ui.json
> @@ -1477,10 +1477,16 @@
>  # @grab-mod: Modifier keys that should be pressed together with the
>  #     "G" key to release the mouse grab.
>  #
> +# @grab-on-tablet: When enabled, the mouse grab is required even for
> +#     tablet (absolute) input devices.  This is useful when the guest
> +#     OS uses a tablet device but you still want click-to-grab
> +#     semantics (e.g. NuttX touchscreen emulation).

Missing: (since 11.1)

When would you still want click-to-grab semantics?  The commit message
gives a fairly specific use case.  The doc comment gives only an
example.  Is this good enough for users?

> +#
>  # Since: 7.1
>  ##
>  { 'struct'  : 'DisplaySDL',
> -  'data'    : { '*grab-mod'   : 'HotKeyMod' } }
> +  'data'    : { '*grab-mod'   : 'HotKeyMod',
> +                '*grab-on-tablet' : 'bool' } }
>  
>  ##
>  # @DisplayType: