In tests, resetting a stream after adding items doesn't remove items

[nathanalderson]

Environment

• Elixir version (elixir -v): 1.14.1 (OTP 25)
• Phoenix version (mix deps): 1.6.15
• Phoenix LiveView version (mix deps): 0.20.0
• Operating system: Linux
• Browsers you attempted to reproduce this bug on (the more the merrier): n/a
• Does the problem persist after removing "assets/node_modules" and trying again? Yes/no: n/a

Actual behavior

In a LiveViewTest, sending items to the stream (via, for example, stream(socket, :some-stream, [item1, item2])) and subsequently resetting the stream (via stream(socket, :some-stream, [], reset: true)) does not remove the items from the DOM.

I have reproduced this in the testbench. See this comparison.

Expected behavior

Resetting a stream should always remove the items from the DOM.

[danschultzer]

FWIW I opened this a while ago: #2677

I don't think this is the right way to solve it, but I couldn't grok the code path to map the phx stream. This was just the simplest fix I could think of.

[nathanalderson]

I updated my fork to test whether the recently merged #2864 would resolve this issue. It does not. The testcase I added to demonstrate the bug still fails in the same way. Updated branch is here, for reference.