Change OOO so that MELS generation will continue over hidden events

[t3chguy]

Fixes vector-im/element-web#4716

this fixes the scenario of N Member events, then an invisible event e.g. (m.room.aliases) then more Member events not forming a MELS or forming one on either side of the invisible event.. Pre hide-join-parts this is how it worked.

Signed-off-by: Michael Telatynski 7t3chguy@gmail.com

[lukebarnard1]

But what happens if the RM is the invisible event? By the looks of it, the RM won't be considered "in" the MELS and it won't get rendered.

[t3chguy]

This was the case and worse from what I can tell pre-hide-join-parts but I'll look into getting around that:
https://github.com/matrix-org/matrix-react-sdk/blob/release-v0.9.7/src/components/structures/MessagePanel.js#L327-L340

[t3chguy]

@lukebarnard1 should be better now

[lukebarnard1]

Ah but what if the hidden event is not a membership event or needs a date separator? Putting the continue before that conditional break feels like it would break things.

[t3chguy]

if it needs a date separator then so will the next one or even if it doesn't, its not being shown in the timeline so why does it matter it needs a date sep?
if it isn't a member event then we don't want it to split a MELS in two
(potentially preveneting either MELS from forming due to min-events)

[t3chguy]

fwiw, my reasoning on the code

we need to mark RM-in-MELS iff
the event is the RM && either of:

• MELS generation loop continued over the event and thus no other code will apply to it
• The event was added to the MELS

so if the break; applies then we should not check if the event is RM, because the outer loop will determine that and we no longer care about that event (otherwise we'd get Two RMs for it)

[t3chguy]

I could reverse the logic, only break if the existing conditions hold and _shouldShowEvent(collapsedMxEv)
to have the break before the continue but the effect would be the same

[lukebarnard1]

if it needs a date separator then so will the next one

Will there be a next one? I think your reasoning is correct, but for this particular edge case, e.g. a membership event or an event that needs a date separator (i.e. true for the break) that isn't being shown for whatever reason (true for the continue before the break) I think it's broken. In this case you would skip over the thing that would otherwise cause the MELs to split in two...

Oh but we don't want it to split it in two.

Then it's fine. It's very very annoying how hard it is to reason about this code, I regret ever writing it like this 😇