fix(rxquery): emit yield/zero synchronously to keep element order

CHANGELOG.md

@@ -12,6 +12,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **Breaking:** `AwaitOperationConfiguration` cases are prefixed with `Await` (`AwaitSequential`, `AwaitParallel 4`, ...) so they no longer collide with `System.Threading.Tasks.Parallel`
 - `mapAsync` and `iterAsync` validate the concurrency limit of the options when they are called
 
+### Fixed
+
+- `rxquery` emitted its elements on the thread pool, out of order and after the source had moved on; `yield` and `zero` are now synchronous
+
 ## [0.3.1] - 2026-01-28
 
 ### Fixed

src/FSharp.Control.R3/Observable.fs

@@ -131,8 +131,10 @@ module Builders =
         member _.SkipWhile (s : Observable<_>, [<ProjectionParameter>] predicate : _ -> bool) = s.SkipWhile (predicate)
         [<CustomOperation("skip", MaintainsVariableSpace = true, AllowIntoPattern = true)>]
         member _.Skip (s : Observable<_>, count : int) = s.Skip (count)
-        member _.Zero () = Observable.Empty (TimeProvider.System)
-        member _.Yield (value) = Observable.Return (value, TimeProvider.System)
+        // Zero and Yield emit synchronously on subscription: scheduling them on TimeProvider.System hopped every element of
+        // a query to the thread pool, so SelectMany merged them in arbitrary order and the results arrived late
+        member _.Zero () : Observable<'T> = Observable.Empty<'T>()
+        member _.Yield (value : 'T) = Observable.Return<'T> value
         [<CustomOperation("count")>]
         member _.Count (s : Observable<_>) = ObservableExtensions.CountAsync (s)
         [<CustomOperation("all")>]