'Unable to open X display' error is produced by a CEF application running in windowless (off screen rendering) mode - chromium-embedded

Question

[01] The application was built and executed in a docker.
The Dockerfile is as follows:
>>>>
FROM ubuntu:xenial
RUN apt-get update -y
RUN apt-get install -y wget
RUN apt-get install -y apt-utils
RUN apt-get install -y python3
RUN apt-get install -y tree
RUN apt-get install build-essential software-properties-common -y
RUN apt-get -y install libgtk2.0-dev
RUN apt-get -y install libgtkglext1-dev
RUN apt-get -y install python3-pip
RUN pip3 install --upgrade pip
RUN apt-get -y install cmake
RUN pip3 install --upgrade cmake
RUN apt-get -y install libnss3-dev
RUN apt-get -y install libxtst6
RUN apt-get -y install libxss1
RUN apt-get -y install libasound2
RUN apt-get -y install libatk-bridge2.0-0
<<<<
the CEF binary being used is
cef_binary_76.1.5+gd8a577c+chromium-76.0.3809.87_linux64/
[02] the application worked fine when starting the docker
with xhost+ and -e DISPLAY=.... as follows:
[A] starting docker with bash
xhost +
docker run \
-it \
--rm \
-e DISPLAY=$DISPLAY
--privileged \
-v /tmp/.X11-unix:/tmp/.X11-unix \
...
--network host \
--name $CONTAINER_NAME \
$REQD_IMAGE \
bash
[B] running the application in bash as
onpaint_streamer \
--no-sandbox \
--disable-gpu \
--disable-gpu-compositing \
--enable-begin-frame-scheduling \
--disable-extensions \
--disable-pdf-extension \
--url=file:///test/027/test.html
[03] the application DID NOT WORK when xhost+ and -e DISPLAY=....
were removed in [02][A]. The following message was dumped
+ ./onpaint_streamer --no-sandbox --disable-gpu --disable-gpu-compositing --enable-begin-frame-scheduling --disable-extensions --disable-pdf-extension --opg-fifo-path=/adhyuh/test/027/onpaint_data_fifo --opg-verbose --url=file:///test/027/test.html
d> CefInitialize : status=about_to
[0812/140301.641476:ERROR:browser_main_loop.cc(1504)] Unable to open X display.
d> CefInitialize : status=done
d> MessageLoop : status=started
[0812/140301.641921:FATAL:context.cc(584)] Check failed: CEF_CURRENTLY_ON(content::BrowserThread::UI).
./run_onpaint_data_writer_in_dk.sh: line 32: 20 Trace/breakpoint trap (core dumped) ./$STREAMER_APP --no-sandbox --disable-gpu --disable-gpu-compositing --enable-begin-frame-scheduling --disable-extensions --disable-pdf-extension --url=file:///test/027/test.html
[04] Some of the other 'flags' embedded in the code are
void SimpleApp::OnContextInitialized() {
//....
CefBrowserSettings browser_settings;
browser_settings.windowless_frame_rate = 30;
CefWindowInfo window_info;
window_info.SetAsWindowless(0);
//....
}
[05] Why should a windowless CEF application require xhost and DISPLAY to be present ? What am i missing or doing wrong ?

Answer 1

CEF depends on X11 library even in windowless mode ... was the answer that i got when a similar post was raised in CEF Forums. Please see https://magpcss.org/ceforum/viewtopic.php?f=6&t=16993&sid=5e46cc6bfb909badf63e0aafcfe9413f&p=42879#p42879