How does LUSID calculate cross gain/loss?

Cross gain/loss is defined as FX gain/loss on market gain/loss. LUSID can calculate cross gain/loss but does not currently report it fully.

Consider the following example, of a single unit of a USD stock in a GBP portfolio that is valued on the purchase date:

Imagine we subsequently value this holding after a change in the market price (from $70 to $88) and the USD/GBP rate (from 0.8 to 0.85):

		Metric	Calculation
Base cost		`Holding/Cost/Pfolio`	Purchase FX rate * Purchase price
Base PV		`Valuation/PvInPortfolioCcy`	Current FX rate * Current price
Base P&L	Total	`ProfitAndLoss/Total/PortfolioCcy`	Base PV - Base cost
	Market	`ProfitAndLoss/Total/Market/PortfolioCcy`
	FX	`ProfitAndLoss/Total/Fx/PortfolioCcy`

However, the market P&L actually consists of two components: market gain, and FX gain on that market gain. It is not currently possible to report this separately in a valuation.

It is possible to examine this separately in journal entry lines if the portfolio is managed by a fund:

Here, the market gain of £15.30 is broken down into:

Core market gain: £14.40
FX gain on the market gain: £0.90 (highlighted in yellow, and reported as both a credit and debit per the double-entry system):

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How does LUSID calculate cross gain/loss?