Layertree popover style inconsistent on Internet Explorer

[pfirpfel]

Affected version: master (and older)

The layertree popover (opacity, show/hide legend, filter) has an inconsistent look on Internet Explorer (reproduced on IE11).

Use the demo as an example.

The first click on the cog opens this popover:

On the second click it looks like this:

On the third and any successive click after that, it looks like this:

In comparison on Firefox, it always looks like this: