fix(windows): run new_window_req_handler on main thread

[Legend-Master]

Implement the solution in #1657 (comment), closes #1657

Run new_window_req_handler on main thread and remove the Send + Sync restriction on it

Also fixed:

• NewWindowResponse::Create not working if no custom protocol's registered
• tauri-runtime-wry's usage of RefCell on another thread pointed out by @sftse in Redraw Sync bounds tauri#14805

[github-actions]

Package Changes Through dc93dcc

There are 1 changes which include wry with minor

Planned Package Versions

The following package releases are the planned based on the context of changes in this pull request.

package	current	next
wry	0.54.1	0.55.0

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[sftse]

Thanks for taking this on.

Because the field is pub, the change is breaking.

pub struct Foo {
    pub f: Box<dyn Fn() + Send + Sync>, // remove Send + Sync breaks the code
}

fn main() {
    let f = move || {};
    let foo = Foo { f: Box::new(f) };
    std::thread::scope(|s| {
        s.spawn(|| &foo);
    });
}

[sftse]

I don't know enough about the code to judge this, but I just briefly sanity-checked whether the function passed is ever Send.
All the call sites pass a function that is not Send, so it might be sensible to drop the second precondition even if it were correct. Judging a more complex precondition is more difficult than a simple one. It also seems like a precondition that would warrant a comment referencing external documentation on PostMessageW, since this is the only unsafe function called in this block.

[Legend-Master]

Not sure what's the better words here 🤔

[sftse]

Would this be incorrect or incomplete? The closures passed are already not Send.

Suggested change

	/// Send `function` to run on `hwnd`'s thread
	///
	/// ## SAFETY:
	///
	/// This function doesn't force a `Send` to make it easier to use,
	/// the caller must call this function on the same thread as `hwnd`
	/// or ensure the function is safe to `Send`
	/// Send `function` to run on `hwnd`'s thread
	///
	/// ## SAFETY:
	///
	/// The caller must call this function on the same thread as `hwnd`.

[Legend-Master]

It's quite tricky here, our custom protocol handler is !Send but marked with unsafe impl Send to send to another thread for async tasks, and then send back and called through dispatch_handler, so technically, it is only accessed on the hwnd's thread but it is not called on hwnd's thread