[draft] detecting close of RBrowser

[linev]

Follow up for #19978

Show how ClearOnClose can be used for RBrowser.
To get signal immediately with browser close,
one should change gEnv->SetValue("WebGui.ReconnectTmout", -1);

[silverweed]

I'm not sure how to translate this for my use case: in rootbrowse.cxx I have this code:

int main() {
   // ... 
 
   auto browser = std::make_unique<TBrowser>();
  
   // For classic graphics: ensure rootbrowse quits when the window is closed
   if (auto imp = browser->GetBrowserImp()) {
      if (auto mainframe = imp->GetMainFrame()) {        
         mainframe->Connect("CloseWindow()", "TApplication", &app, "Terminate()");
      }
   }
  
  // TODO: do the same as the paragraph above but for the web graphics

   std::cout << "Press ctrl+c to exit.\n";
   while (!gROOT->IsInterrupted() && !gSystem->ProcessEvents()) {
      std::this_thread::sleep_for(std::chrono::milliseconds(10));
   }
   return 0;
}

Ideally I don't want to "know" about the internal RBrowser from here, I want to talk to browser or browser->GetBrowserImp().
I want to call TApplication::Terminate() when the ClearOnClose is triggered, but how do I do it from here? (without modifying the TBrowserImp interface further?)

[github-actions]

Test Results

    21 files      21 suites   3d 15h 41m 53s ⏱️
 3 678 tests  3 677 ✅ 0 💤 1 ❌
75 420 runs  75 414 ✅ 5 💤 1 ❌

For more details on these failures, see this check.

Results for commit 1327f51.

[linev]

Ideally I don't want to "know" about the internal RBrowser from here, I want to talk to browser or browser->GetBrowserImp().

That's why I add last commit with changes of RWebBrowserImp class.
It can assign ClearOnClose handle which is destroyed when last connection is closed.
And provide its own handler here.

So PR is incomplete and just shows the direction.
One still need add some handler to base TBrowserImp class and invoke it in correspondent implementation properly.

Also one should be careful - ClearOnClose handle destroyed also during regular ROOT shutdown when instance of RWebBrowserImp can be already deleted.

[silverweed]

To get signal immediately with browser close, one should change gEnv->SetValue("WebGui.ReconnectTmout", -1);

Can't we add a SetReconnectTmout instead of having to pass through TEnv (which does an insane amount of work to Set/Get values compared to what could be in principle done by setting a single class member)?
Also, as far as I understand it's currently impossible to change this timeout after having created the RWebBrowserImp, which is not ideal