Vanishing sidebar in IE11

[maxime-rainville]

Affected version

Tested in SilverStripe 4.3

Description

Using IE 11, after you navigate pages in the CMS with the left nav set to auto collapse, the left nav sometimes goes away entirely. Other admin areas seem unaffected

If you fix the nav in open position, you are fine.

Steps to reproduce

• Access any Page in the CMS.
• Navigate to an other page.

https://youtu.be/mD40YjdnWcE

[Cheddam]

Several IE 11 layout fixes have been merged over the last few days, so it'd be worth re-testing whether this is an issue @maxime-rainville (I can't reproduce the issue myself).

[maxime-rainville]

I was still seeing it last week, but your PR may have fixed it.

[maxime-rainville]

We've adjusted other part of how this panel works.

I can't reproduce this issue in IE11 any more using SS4.6 core or SS4.6 kitchen sink.

We'll call this one fixed.