How to support mixed DPI in Xwayland?

Joseph Burt caseorum at
Sun Sep 10 20:25:59 UTC 2017

On Sun, Sep 10, 2017 at 5:01 PM, Joseph Burt <caseorum at> wrote:
> Hi Olivier,
> On Thu, Sep 7, 2017 at 6:17 PM, Olivier Fourdan <ofourdan at> wrote:
>> The other solution would be to have the same screen, but have Xwayland to
>> give different scaling conversions for root window size, screen size, events
>> coordinates, etc. depending on the client, if it's HiDPI aware or not,
>> some sort of a "hidden" screen.
> I'm not aware of a facility for X clients to advertise DPI awareness
> to the X server. What would the heuristic be? Or just user-specified
> scaling for a specific X client?
> On scaling: DPI awareness isn't really possible in Wayland at the
> moment. The client needs to be told what DPI the compositor would like
> it to render, and must be able to acknowledge that. Pretty close to
> the current spec would be to have wl_output::scale and buffer_scale
> non-integers with an explicitly defined pixel size corresponding with
> 1 (96, 72 or 75 DPI, 0.25mm/pixel, whatever), but there are so many
> possibilities. Even just the client attaching a differently-sized
> buffer than it's surface "size" in such normed pixels would be
> acknowledgment enough.
> What's the current state of that conversation?

I'm now up to date on the relevant bug reports. Sorry for the spam.

What about always running the X server at hardware resolution,
limiting the size of lodpi clients on the XWM side, and letting the
compositor scale them?


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