[Webkit-unassigned] [Bug 185764] New: [WPE] Rendering on a HiDPI display looks scaled up instead of rendered at 2x

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=185764

            Bug ID: 185764
           Summary: [WPE] Rendering on a HiDPI display looks scaled up
                    instead of rendered at 2x
           Product: WebKit
           Version: Other
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKit WPE
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: aperez at igalia.com
                CC: bugs-noreply at webkitgtk.org

Created attachment 340688

  --> https://bugs.webkit.org/attachment.cgi?id=340688&action=review

Comparison of WPE and GTK+ ports rendering the same page on HiDPI

Running the Cog WPE launcher (https://github.com/Igalia/cog) with
the WPE FDO backend (https://github.com/Igalia/WPEBackend-fdo) in
a Wayland compositor which is configured for 2x scaling on a HiDPI
display, the resulting output looks like it was rendered at 1x and
then its size doubled (scaled up by the compositor, I think). The
expected behaviour would be to render at 2x.

For comparison, I am attaching a screenshot of the same webpage
side by side: Cog (WPE) on the left, and Epiphany (WebKitGTK+) on
the right.

Also, note that the compositor being used is the wlroots branch
of Sway (http://swaywm.org), but I doubt that's the cause. Note
that Cog will print out the following to standard output:

  % cog -P fdo perezdecastro.org
  Wayland: Got a wl_compositor interface
  Wayland: Got an xdg_shell interface
  Wayland: Got a wl_seat interface
  EGL initialized with version 1.4
  Seat name 'seat0'
  Seat caps: Pointer Keyboard Touch
  New XDG toplevel configuration: (0, 0)
  Cog-Message: 12:04:54.118: <http://perezdecastro.org/> Load started.
  Cog-Message: 12:04:54.119: <https://perezdecastro.org/> Redirected.
  New XDG toplevel configuration: (636, 695)
  ...

Note the 636x695 size for the Cog window/surface. The physical resolution
of the screen is 2560x1440, which translates into the 1280x720 logical
resolution, as Sway tells us: 

  % swaymsg -t get_outputs
  Output eDP-1 'Sharp Corporation LQ133T1JW19 0x00000000'
    Current mode: 1280x720 @ 59.970001 Hz
    Position: 0,0
    Scale factor: 2x
    Transform: normal
    Workspace: 1
    Available modes:
      2560x1440 @ 39.999001 Hz
      2560x1440 @ 47.918999 Hz
      2560x1440 @ 59.970001 Hz

So I *think* that the compositor is working correctly, and there has to be
something missing either in WPEBackend-fdo, in Cog, or WebKit (maybe some
changes are needed all across!) in order to use high-resolution rendering
when Wayland tells to the client that a scaling factor is being used for
a surface.

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