[Bug 60555] New: winex11.drv: OpenGL child window rendered offscreen shows the previous frame until the application presents again (EGL, Xwayland)

[WineHQ Bugzilla]

http://bugs.winehq.org/show_bug.cgi?id=60555

            Bug ID: 60555
           Summary: winex11.drv: OpenGL child window rendered offscreen
                    shows the previous frame until the application
                    presents again (EGL, Xwayland)
           Product: Wine
           Version: 11.20
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: winex11.drv
          Assignee: [e-mail]
          Reporter: [e-mail]
  Target Milestone: ---
      Distribution: ---

An application that draws its OpenGL viewport in a child window and only
redraws on input (Autodesk Fusion 360) always
shows the frame before the last one. Hovering a vertex or face draws the
highlight, but it only appears on screen when the
next input (a click, the wheel) makes the application present another frame. A
rubber-band rectangle stopped halfway
lags one step behind the cursor. Reports from other users with the same symptom
on AMD and NVIDIA cards:
https://codeberg.org/cryinkfly/Autodesk-Fusion-360-on-Linux/issues/686

Environment: Arch Linux, wine 11.20 (Arch package), winex11.drv with the
default EGL backend, Hyprland 0.56 with
Xwayland 24.1.14, NVIDIA RTX 5080 with driver 615.78.

What I observed:

- With WINEDEBUG=+timestamp,+wgl and the cursor position logged on the same
clock: while the cursor rests on a vertex for
  4 seconds there is no GL call at all. The application had already drawn and
swapped the frame with the highlight on
  the last mouse move, but the screen shows the frame before it.
- Not affected by __GL_SYNC_TO_VBLANK=0 or by UseEGL=N (GLX path; this driver
has no GLX_OML_sync_control). I have only
  tested Hyprland's Xwayland, not a plain X server.

Analysis (from reading dlls/winex11.drv/opengl.c and init.c at wine-11.20 and
current master):

The viewport is a child window, so needs_offscreen_rendering() always returns
TRUE ("child window, needs compositing")
and the client window is redirected. After eglSwapBuffers(),
x11drv_egl_surface_swap() only calls XFlush() and then
client_surface_present(), which immediately copies the redirected window into
the top-level window
(X11DRV_client_surface_present -> StretchBlt). Nothing waits until the X server
has applied the swap, and with a swap
interval of 1 Xwayland applies it at the next frame, so the copy reads the
previous contents. The GLX path waits with
glXWaitForSbcOML() when GLX_OML_sync_control is available, but the EGL path has
no equivalent.

As an experiment, making the offscreen swap complete before the copy (swap
interval 0 for offscreen surfaces, glFinish()
before eglSwapBuffers() and XSync() after it) makes the highlight and the
rubber band follow the cursor exactly. I am not
attaching that change: it was written with an AI assistant, which the Developer
FAQ does not accept. A proper fix
probably needs to wait for the presentation to complete, or to copy again once
it has.

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