The JS code running for the toolbar is expected to have the widest browser support possible, because this runs in the app page directly, and we don't want those pages to be restricted in their browser support just because the Symfony debug toolbar breaks the JS.
Our toolbar code is currently still compatible with IE, and this PR would break it (note that I'm specifically talking about the toolbar, not about the web profiler UI which renders its own pages)

Thanks for the information.

My bad, I thought that IE was unsupported for 5+ years, since Edge was released.

@alexislefebvre lots of enterprise tools still need to support IE due to their user base (though this is gradually going down, thanks to Microsoft pushing for the end of support of IE11).
And as Symfony has no knowledge about the intended browser support of projects, the web debug toolbar should avoid forcing to reduce that range by causing errors. for the web profiler UI, we already dropped support for IE 11 a while ago.

My bad, I thought that IE was unsupported for 5+ years, since Edge was released.

IE 11 is still supported until June 2022 on Windows 10 (after which the IE mode of the new Edge will be the only supported way to run a page needing IE on Windows 10) and until the EOL of Windows 8 (which is in 2023) on Windows 8.

Thanks for the detailed report.

@stof can we reopen this PR now that IE is unsupported? 🙂 https://docs.microsoft.com/en-us/lifecycle/announcements/internet-explorer-11-end-of-support

Update: my bad, you already answered:

and until the EOL of Windows 8 (which is in 2023) on Windows 8