Clicking "Pop out" on two different views causes the latter to replace the first

[rantahar]

To reproduce:

• Open index
• Click "Pop out" next to "Big control panel" -> new window opens
• Click "Pop out" next to "Real-time scene preview" -> the preview replaces the control panel

Not a high priority, it's easy enough to open one in a new window manually.

[rkdarst]

Good catch - I submitted an initial fix, and it seems to work, but I should sometime verify that everything works as expected.