Bug 291217 – REGRESSION(292977@main): [Win] nested tasks are not dispatched

• Component: Platform
• Status: RESOLVED FIXED

[Yury Semikhatsky]

After https://commits.webkit.org/292977@main nested tasks on main thread are not dispatched. 

Try inserting the following code somewhere in the UI process, e.g. in `WebPageInspectorController` constructor. `step 2 callback` is printed, but `step 3 callback` is not.

```
fprintf(stderr, "step 1\n");
RunLoop::main().dispatch([] {
    fprintf(stderr, "step 2 callback\n");
    RunLoop::main().dispatchAfter(Seconds(0.1), [] {
        fprintf(stderr, "step 3 callback\n");
    });
});
```

It broke Playwright screencast and video recording functionality.

[Fujii Hironori]

The run loop of UI process main thread is run by the application, not by WTF::RunLoop::run.
https://github.com/WebKit/WebKit/blob/2bebfdc1ea6d02362f3b549004940f2ef8ea28e2/Tools/MiniBrowser/win/WinMain.cpp#L81-L91

I think we should revert 292977@main.

[Fujii Hironori]

Created attachment 474861 [details]
test patch for MiniBrowser

[Ian Grunert]

Yeah, this used to work because the main thread's run loop and the WTF run loop both processed the WM_TIMER messages.

Agree we should revert for now. We might be able to do something similar to PlayStation's MiniBrowser and use the WTF run loop in MiniBrowser, but I'd rather revert and we can land that later.

[Fujii Hironori]

(In reply to Ian Grunert from comment #3)
> Agree we should revert for now. We might be able to do something similar to
> PlayStation's MiniBrowser and use the WTF run loop in MiniBrowser, but I'd
> rather revert and we can land that later.

The run loop is a bit customized.
https://github.com/WebKit/WebKit/blob/2bebfdc1ea6d02362f3b549004940f2ef8ea28e2/Tools/MiniBrowser/win/WinMain.cpp#L85-L86

Another customize is needed if we add a modeless dialog.
https://learn.microsoft.com/en-us/windows/win32/dlgbox/using-dialog-boxes#creating-a-modeless-dialog-box

[Fujii Hironori]

293380@main reverted 292977@main.

[Radar WebKit Bug Importer]

<rdar://problem/148773454>