feat(webview2): add with_native_window_occlusion option

[podheitor]

feat(webview2): add with_native_window_occlusion option

What

Adds a Windows-specific builder option WebViewBuilderExtWindows::with_native_window_occlusion(bool) that lets an app opt out of WebView2's native window occlusion detection.

When set to false, wry appends CalculateNativeWinOcclusion to the --disable-features list it already passes by default. It defaults to true, so current behavior is unchanged unless you explicitly opt out.

Why

On Windows, WebView2 stops (or never starts) rendering a webview whose host window is hidden or fully occluded, to save resources. That's fine for a single foreground webview, but it breaks apps that pre-create several webviews and show/hide them on demand (tabbed browsers, multi-service messaging shells, etc.): a webview created while hidden may never bring up its render surface and paints blank the first time it's shown.

This is a long-standing pain point with no in-library workaround today:

• [bug] new webview added to the window does not appear on top (Windows) tauri#9798 — added webview isn't rendered/on top (Windows)
• [bug] Additional WebviewWindow Shows Blank Screen When Using additionalBrowserArgs in tauri.conf.json tauri#13092 — additional WebviewWindow blank
• WebView2 windows are blank when created in the background MicrosoftEdge/WebView2Feedback#1077 — WebView2 created in a background/not-shown window renders blank
• Problem when changing WebView2 control’s parent visibility MicrosoftEdge/WebView2Feedback#1094 — gray after parent hidden → visible

Disabling CalculateNativeWinOcclusion is the documented lever for this, but the only way to reach it today is with_additional_browser_args, which fully overrides wry's default args (the mini-menu / SmartScreen / autoplay / proxy flags) — so you lose all of those unless you reproduce them by hand. This option exposes just the occlusion toggle while keeping wry's defaults intact.

How

• New field native_window_occlusion: bool on the Windows PlatformSpecificWebViewAttributes (defaults to true).
• New trait method on WebViewBuilderExtWindows.
• In webview2/mod.rs, when the option is false, append ,CalculateNativeWinOcclusion to the default --disable-features string. No effect when with_additional_browser_args is used (documented), since that path is a full override.

Usage

let webview = WebViewBuilder::new()
    .with_url("https://example.com")
    .with_native_window_occlusion(false) // keep rendering while hidden/occluded
    .build(&window)?;

Notes

• Windows-only; no-op on other platforms (the method only exists under #[cfg(windows)], like the other WebViewBuilderExtWindows options).
• Type-checks for x86_64-pc-windows-gnu.
• Added a covector changelog entry (minor).

Real-world validation: this is what made reliable multi-webview show/hide work in a Tauri app that pre-creates one hidden webview per service and swaps them — without it, the second-and-later webviews paint blank on first show.