Let's do something to make the manual test sidebar less annoying

[oleq]

📝 Provide a description of the improvement

ATM the sidebar is responsive. When the viewport width is less than a fixed number, the sidebar collapses to the left. There are some problems with this approach:

• Sometimes you have dev tools (e.g. console) open on the right side of the screen and you work with the editor. When you try to re-scale it, for instance, to see a lengthy log, you find out that changing dimensions of the dev tools causes the sidebar to disappear. As a result, the editor content becomes wider and there's a sudden move of the content and the place you were interested in is now hundreds of pixels up the screen.
• Now the same scenario in the opposite direction: your content was narrow, then you scale dev tools down (to the right) and suddenly the content becomes very wide. 
• ☝️ These are especially annoying when you debug things like dynamic toolbar grouping. You want to scale the editor up or down fluidly instead of sudden geometry change related to the manual test sidebar.
• You have to hover the mouse over the sidebar to reveal it. Some of us have docks that show up on hover (like in MacOS, or whatever UI in other OSs) on this side of the screen or just another screen right next to this side of the web browser. I can't tell you how many times I performed the wrong action...
• The collapsed sidebar looks just weird (a grey bar of 10-20px taking up the entire side of the screen).

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[oleq]

I proposed the following DX/UX in the PR: