WIP alternative to#16165. The real fix, I think, is to not use effects for synchronization at all, but rather to invalidate indirect bindings on mutation. In other words in a case like this...

<script>exportlet selected;exportlet tasks;</script><selectbind:value={selected}>  {#eachtasksastask}    <optionvalue='{task}'>{task.description}</option>  {/each}</select><label>  <inputtype='checkbox'bind:checked={selected.done}> {selected.description}</label><h2>Pending tasks</h2>{#eachtasks.filter(t=>!t.done)astask}  <p>{task.description}</p>{/each}

...updatingselected should also updatetasks, because the bindings are linked. I think we might be able to use this mechanism for each blocks as well and end up with simpler compiler code, though I don't have time to finish it right now and my brain needs a rest anyway because this stuff is confusing as hell. Can't wait to be able to delete all this legacy gubbins.

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