iTwin
Fix viewport context loss by changing event listener
When the window.location.href of the viewer tab is modified, but the link is not a valid one, we still see the viewport context disappear, because IModelApp.shutdown() is called beforeunload (i.e regardless of whether a user is redirected, or a tab is closed, or not). Changed the event listener to be a unload event, but this is not intended to be the long-term solution, because it is deprecated
I explored using the visibilitychanged event, but the problem with that is IModelApp will be shutdown when a user switches to a different tab. Checking the window.visiblityState and whether it is visible or hidden is the way to do it, but browsers aren't able to differentiate whether a hidden visibilityState is from the browser closing/refreshing a tab or because it is switched. So in conclusion, there is no good long term solution right now...
Long term we should still look into switching over to the visibility APIs for cleanup:
- Better resource management when multiple tabs are opened.
- Unreliable
beforeunload- depending on what is done in the handler we might lose user/app state https://www.igvita.com/2015/11/20/dont-lose-user-and-app-state-use-page-visibility/
