Use singleton object for Float x/ycursor

[Carreau]

Attempt to start implementing #182

I'm not familiar with the codebase so it might not be he right way to do that,
I suppose the all logic here could be simplified and keep backward compat with (x|y)cursor

Screenshot with this as a completer:

Float(left=cursor+7,
       top=cursor-1,
                      ...))

(note, my goal is to have a completer like Fish which is a cursor.y+1 but the all width of terminal in the end, but not really a hsplit)

[jonathanslenders]

Hi @Carreau,

Thanks a lot for the issue and the work on the pull request!

My thoughts:

• In general, I very much like the idea of the Cursor singleton and the __add__/__sub__ overload.
• However, this is still not going to be flexible enough. The current algorithm will position the content of the float above the cursor when the input spans multiple lines. In that case, saying top=cursor-1 is incorrect.

Possibly a better approach is to rewrite the FloatContainer.write_to_screen function and use a more powerful interface for plugging in other float positioning algorithms.
Ptpython for instance, also uses the BufferControl.menu_position to display the function signature at the start of the function call. (https://github.com/jonathanslenders/python-prompt-toolkit/blob/master/prompt_toolkit/layout/controls.py#L457) I would like to get rid of that, if we find a better, more generic approach.

For this, I also don't mind to be backwards-incompatible (it's mostly used internally), but I prefer to do it correctly.

Currently I'm still abroad for work with few spare time, but I will think about this. It's definitely interesting!

Jonathan

[Carreau]

In that case, saying top=cursor-1 is incorrect.

Yeah, that was just an example to see that minus was working definitively not good for UI.

For this, I also don't mind to be backwards-incompatible (it's mostly used internally), but I prefer to do it correctly.

I'm still wrapping my head around the different case here. Especially xcusrsor and ycursor are hints for positioning. I'm trying to find the correct abstraction. To simplify the interface.

I'm also pretty busy with IPython these days, I was mostly trying to change my mind a bit, but will try to work a bit on that and understand more of the code.