make tab visible in codemirror and light red background

[Carreau]

Shoudl fix #3935

before merging, I would like to find out why there is one line which is suppressed in IPython/html/static/style/ipython.min.css

[Carreau]

On top of #3938

[arijun]

The heavy blocks might look a bit overbearing but should do the trick. If cut and pasted back out of the notebook, would the indicators be copied as well, or would they be converted back to tabs? Probably the latter is more desirable.

[minrk]

@arijun the tabs are not replaced by anything, this is just the CSS styling of the tabs. So copy/paste should work normally.

@Carreau red warning might be a bit much, maybe just the symbol and no background?

[minrk]

marking for 2.0.

[Carreau]

red warning might be a bit much, maybe just the symbol and no background?

I found the symbol only a little too discreet, especially if you have only one. And as you should never have tab in notebook (for Python at least) user should never see the red. Also for me just the symbol would not mean that something is wrong.

But I have no strong feeling against removing he red. I`ll try to see if I can make the arrow red, without background.

[minrk]

I think just the symbol is pretty clear:

[damianavila]

Red background is too much, having into account the "light" theme of the notebook, red blocks are disturbing, I think...

[Carreau]

I'll remove it then. But keep in mind that there should never be tab in a notebook cell. So at best users will never see that.

[Carreau]

Red removed, and rebased.

[damianavila]

Great