Desktop app is just a black box

[bobosola]

Egui complete beginner here, and struggling. I tried to extricate /examples/hello_world_simple from the workspace in order to try build the simplest possible stand-alone version locally. So my Cargo.toml is :

[package]
name = "egui_test"
version = "0.1.0"
edition = "2021"

[dependencies]
egui = "0.28.1"
eframe = "0.28.1"

I then copied main.rs (with the env_logger line commented out). It builds and runs, but just gives me a black window with only the title bar showing. I'm on Lubuntu and have installed the extra requirements for eframe:
sudo apt-get install libxcb-render0-dev libxcb-shape0-dev libxcb-xfixes0-dev libxkbcommon-dev libssl-dev though I have no idea what they do or even if they are needed? All the examples here (and on other egui demo sites) are all embedded in a complex tree of relationships with Cargo.toml workspace entries which are defeating me in getting a simple stand-alone app running. I'd be very grateful for any help in getting started please.