[windows] fix memory leak in WebView

[jonathanpeppers]

Fixes: #20283
Fixes: #22972
Context: https://github.com/davide-cavallini/webview-winUI-maui-leak

The above sample on Windows was leaking because it does the following:

1. Your app has a single Window

2. Navigate to a page with a WebView

3. Navigate away

Because the Window remains alive, WebViewHandler subscribes to Window.Closed which keeps a reference to the WebView and keeps it alive indefinitely.

I was able to reproduce this in a test, that keeps the Window alive before calling AssertionExtensions.WaitForGC().

The solution was to move the Window.Closed subscription to the WebView2Proxy nested class. This makes sure that the WebView, its handler, etc. can be collected before the Window is closed.

I also found a secondary issue while debugging, if you call:

webView.Close(); // MauiWebView or WebView2

If CoreWebView2 is not initialized, it will throw a C++ exception.

We can instead do:

if (webView.CoreWebView2 is not null)
{
    webView.Close();
}

[jonathanpeppers]

There is a Windows test failure, but not sure if it's related to my changes. I reran it again.

[rmarinho]

Failing tests not related.

[bronteq]

Hi @jonathanpeppers, I tried the repro project in the following way

• Release and Debug mode
• Set Microsoft.Maui.Controls 8.0.80-ci.net8.24374.2
• Set Microsoft.Maui.Controls.Compatibility 8.0.80-ci.net8.24374.2

For me the bug does not seem solved, in fact I don't see in Task Manager the memory free up, the situation seems the same as reported in the bug.

If the following row is added in OnDisappearing(), the memory is freed also in Task Manager
webView.Handler?.DisconnectHandler();

Am I doing something wrong? 8.0.80-ci.net8.24374.2 seems the latest nightly version available for the moment

[jonathanpeppers]

@bronteq the fix here prevents every Window from keeping the WebView and MauiWebView (platform view) from staying alive until the Window is closed.

If there are msedgewebview2.exe staying alive, this is probably a Windows App SDK (or WebView2) issue. I know some of this is intentional, as they keep the WebView2 process alive so it can be reused:

• WebView2 leaks old msedgewebview2.exe processes with every page change microsoft/microsoft-ui-xaml#9088

If calling DisconnectHandler() fixes something for you, I would keep using it. In .NET 9, they are going to make this be called more frequently, so it will likely fix this issue long-term:

• Add HandlerProperties.DisconnectPolicy for controlling when DisconnectHandler is called #23738

[bronteq]

Sorry @jonathanpeppers I misunderstood your PR.

My initial issue was #21194
This issue was considered a duplicate of #20283 that should be solved with this PR.

If the issues are different, probably my issue should be reopened, because it is not solved yet and the amount of ram that is not freed up can be very high (think of an industrial pc with limited specs, the program becomes unusable).

Calling DisconnectHandler() solves my issue, good to hear of #23738

[RuddyOne]

I am still seeing the issue on 8.0.80. Am I missing something?