Output TOC collapses expanded nodes on topic click

[RiverMonster]

With our HTML5 output, suppose a user expands some TOC nodes in the left nav. When they then go on to click a topic the topic opens (as expected) but the TOC reloads with those previously opened nodes now collapsed.

There are two reasons why this behavior can be irritating...i. As it happens in a flash, there is the sense that something in the TOC structure has changed, but it is not immediately clear to the user what that is ii. The user may need to expand the nodes again to see the previously displayed TOC entries.

Is there a way to control this (that is, if the TOC needs to reload then it does so with the same nodes expanded as before)?

I optimistically thought it might be affected by the "Synchronize navigation elements with TOC entries" target setting, but it is not. I'm guessing this would need to be done by adding suitable code to one of the JS files? Has anybody done this?

Thanks

Adrian