Most Wasm demos broken

[ogoffart]

Bug Description

Most wasm demo are either not working at all or having the wrong size.

For example

https://snapshots.slint.dev/master/demos/opengl_underlay/ broken since 1.12.0

https://snapshots.slint.dev/master/demos/plotter/ wrong size - broken since 1.12.0
https://snapshots.slint.dev/master/demos/todo/ same

https://docs.slint.dev/latest/demos/slide_puzzle/ wrong size - broken since 1.13.0

Reproducible Code (if applicable)

Environment Details

• Slint Version: Most demo broken since 1.12.0
• Platform/OS: Wasm: Firefox / Chromium on linux

Product Impact

No response

[tronical]

I can reproduce the wrong size issue. Oddly, the plotter looks good to me, but the others are clearly wrong. I made a PR for the panic in the OpenGL underlay example. As that fixes rendering, the wrong size issue shows up there, too.

[tronical]

AFAICS this is what seems to happen, for example with the todo example:

1. We create a new winit window with a min size of 304/172 (logical) and a sufficiently large max size, but no inner size set.
2. Then the code below in set_visible() gets invoked:

                #[cfg(target_arch = "wasm32")]
                if let Some(html_canvas) = winit_window.canvas() {
                    let existing_canvas_size = winit::dpi::LogicalSize::new(
                        html_canvas.client_width() as f32,
                        html_canvas.client_height() as f32,
                    );
                    // Try to maintain the existing size of the canvas element, if any
                    if existing_canvas_size.width > 0. {
                        preferred_size.width = existing_canvas_size.width;
                    }
                    if existing_canvas_size.height > 0. {
                        preferred_size.height = existing_canvas_size.height;
                    }
                }

This overrides the layout's (correct) preferred size with 344/172.

This logic has been there forever.

[tronical]

The slide puzzle is still broken :(