Crash when minimising the "Window" widget

[stephanbuys]

Hi,

When I run the ui.rs example and I "minimize" the "Window" widget I consistently get the following crash ( https://github.com/mvlabat/bevy_egui/blob/main/examples/ui.rs#L110):

wgpu error: Validation Error

Caused by:
    In a RenderPass
      note: encoder = `<CommandBuffer-(1, 284, Metal)>`
    In a set_scissor_rect command
    Invalid ScissorRect parameters


thread 'main' panicked at 'Handling wgpu errors as fatal by default', /.../.cargo/registry/src/github.com-1ecc6299db9ec823/wgpu-0.7.1/src/backend/direct.rs:1896:5
stack backtrace:
  11: <bevy_egui::egui_node::EguiNode as bevy_render::render_graph::node::Node>::update
             at ./src/egui_node.rs:266:9
          at ./examples/ui.rs:7:5

[mvlabat]

Hi @stephanbuys! Thanks for creating the issue. Yeah, I was able to reproduce it while working on the 0.4.0 release as well. I believe that Egui started sending ScissorRect with negative height, which is problematic for some graphics backends. I think that this is a problem on the Egui side, though I'm not 100% sure about this. I was going to reach out to the Egui author about this, but I haven't got a chance yet.

If you'd like to investigate this deeper and probably report this problem in the Egui repo, I'd appreciate this. But anyway, I was going to do that myself at the weekend.

[stephanbuys]

Ok, so I "patched" the file to see if I could do a hotfix as follows (https://github.com/mvlabat/bevy_egui/blob/main/src/egui_node.rs#L313)

This "patch" fixes the crash.

                    let scale_factor = window_size.scale_factor * egui_settings.scale_factor as f32;

                    let (x,y,w,mut h) = (
                        (draw_command.clipping_zone.min.x * scale_factor) as u32,
                        (draw_command.clipping_zone.min.y * scale_factor) as u32,
                        (draw_command.clipping_zone.width() * scale_factor) as u32,
                        (draw_command.clipping_zone.height() * scale_factor) as u32
                        );

                    log::info!("{:?}, {:?}, {:?}, {:?}", x,y,w,h);

                    if h == 0 {
                        h = 1;
                    }

                    render_pass.set_scissor_rect(
                        x,y,w,h
                    );

So as a total newbie just playing this through my mind logically I think it would be impossible to render something of zero height or width, so I would suggest that if a egui widget is set to (w,h) = (0,0) rendering should be skipped for that widget? Logically it still exists in Egui, but in terms of rendering it it is pointless.

[stephanbuys]

@mvlabat I submitted my suggested fix as PR #16 I don't think it's a egui issue, I wasn't able to induce any negative numbers.

[mvlabat]

I've just released the 0.4.1 version with the fix. Thanks for the contribution!