Fixed confirmExit behaviour for Electron apps

[mcgordonite]

What it does

This PR restores the confirmation dialog shown in Electron mode when the user tries to exit without saving changes. On master, there is no confirmation, the app exits without saving.

This relates to an existing Electron issue: electron/electron#9966. Not opening a dialog in the beforeunload handler and using the will-prevent-unload event gets around this.

How to test

Start the Electron app then disable Auto Save. Either:

• Set the "application.confirmExit" preference to "ifRequired", make a change in a file but do not save it.
• Set the "application.confirmExit" preference to "always".

Then try:

• Opening a new workspace.
• Refreshing the page.
• Closing the app.

In these cases, a confirmation dialog should be displayed.

• Clicking "Yes" should close the modal and unload the window.
• Clicking "No" should close the modal without unloading.

In all other cases, the app should behave as before with no confirmation modal.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[kittaakos]

I am verifying this...

[kittaakos]

On master, there is no confirmation, the app exits without saving.

It works for me. See the commit hash and the behavior. Do not get me wrong, I do not mind switching to your solution, but I want to reproduce the problem fist. What should I do to not get notified before closing the application? Thanks!

ifRequired:

always:

[mcgordonite]

@kittaakos that's interesting, I can reproduce it every time. I have tried it on:

• Windows 10 (1809), node 10.16.1.
• Ubuntu 19.04, node 10.16.3.
• My colleague's mac.

I start from a fresh Theia clone, then:

1. Checkout 24f5d20
2. yarn
3. yarn rebuild:electron
4. cd examples/electron
5. yarn start

Then I follow the steps in the PR description.

For what it's worth, here's Ubuntu 19.04 with always:

Windows 10 with ifRequired:

One possible difference is that I use the close button in the title bar, but it also happens when I use the ALT-F4 shortcut.

[kittaakos]

I can reproduce it every time

I will try it on my Windows image.

[kittaakos]

I will try it on my Windows image.

The master state is broken on Windows so I am going to review your changes and merge if it is OK.

[kittaakos]

I think the default Chrome message is: https://github.com/eclipse-theia/theia/pull/6285/files#diff-b23416bfa30b320828382edc2ba8ea30L35
Do we want to align electron with Chome?

[kittaakos]

I have verified it on my Windows image; it worked nicely.

Thank you for your contribution, @mcgordonite 👍