[Feature Request] Allow pre-filling of order quantity to close entire position

[ndorf]

Is your feature request related to a problem? Please describe.
It's somewhat related to #424, wherein imprecisely displayed share positions make it difficult to fully close one's entire position even manually, but ultimately orthogonal.

Describe the solution you'd like
Most trading platforms support some way of easily prefilling a sell order with one's entire current position. In the Augur UI, I think it'd be best to either simply allow clicking on the "Position" row, or, alternatively, have a per-row "Close" button there (similar to the "Cancel" button in the "Open Order" row).

Specifically, it would simply set the order entry form's type to "Sell" for long positions or "Buy" for short ones, set the currently selected outcome to be the one associated with the position, populate the order's quantity with the position's quantity, and leave the limit price field unchanged or blank.

Describe alternatives you've considered
Examples from other trading interfaces I've used recently include:

1. pro.coinbase.com presents a "max" button associated with the order quantity field. Perfectly reasonable, but more complicated: it would also have to work for opening new positions, in which case 1) the limit price would already have be filled in, since it's required to know the quantity of any new shares, 2) the quantity would also depend on the current gas price and 3) it wouldn't be exact anyway, since you never know the final gas cost. In my opinion it is simpler to handle closing existing positions only.

2. kraken.com shows "available amount" near the order quantity field, which populates the field with itself when clicked. In my opinion, this is kind of redundant since the position quantity is already shown on the same screen (albeit not correctly per [BUG]Rounded or truncated position/quantity makes selling all shares difficult #424), but might be easier to implement since there'd be no need to automatically change the currently selected outcome.

3. fidelity.com has "Sell All Shares" as a third order type alongside "Buy" and "Sell" which more or less works with their UI but doesn't seem to make much sense for Augur's.

[adrake33]

@sharkcrayon @matt-bullock What are your thoughts on how we should handle this in the UI? We should make it clearer exactly how many shares the user has, but we may want to do this by allowing the user to hover over the shares and see the more precise amount. Would also be good to have some way for the user to buy/sell all shares to close their position.

[adrake33]

I talked to @Chwy5 about this. We both like the idea, but it is not very high priority right now. We'll probably need to think through this more thoroughly, too, particularly when reversing a position, because there are both shares and ETH involved. Moving this to the backlog for now.

[bthaile]

User just wants to pre populate the form by clicking on a value in their position.

I can see that this could be difficult because of synthetic positions. I think just giving them ability to populate quantity by clicking on their existing position and make sure to switch the buy | sell would go far.

[adrake33]

That sounds good. As Joey mentioned in Discord, we can allow the user to click on the quantity of shares they own. If positive, put the focus on the sell window with the full precision amount. If negative, put the focus on the buy window with abs(position) as the amount.

[Chwy5]

This is fine for now.....but at some point I want to add were we have tabs the say 25%, 50%, 75%, 100%...which allow the user to quickly submit a portion of their total available trading balance.....Just wasnt going to add it for the initial trading pg. launch