This issue also exists in the latest rolling release. cc @coeuvre 

Also seeing this after upgrading to 4.2.1 from 4.0.0. These are the changes that have gone in since the 4.1.0 release:
```
$ git log HEAD ^4.1.0 --oneline src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
bf5e2b8ec0 Remote: Async upload (Part 8)
b51b31dbe7 Remote: Fix a race when reporting action progresses.
0f812eb5e5 Remote: Display download progress when actions are downloading outputs from remote cache.
f0983dfddb Remote: Report checking cache status before the action is scheduled to run remotely.
```

It looks like there was an attempt at fixing the UI bug in `b51b31dbe7`.

By adding the below patch you can reproduce this issue:
```
id 'File:[[<execution_root>]bazel-out/k8-opt/.../foo' is already finished, and event Downloading foo, 5.2 KiB / 5.2 KiB is being processed
...
```

The problem seems to be like @DavidANeil mentioned -- `actionProgress(ActionProgressEvent event)` is probably called in ThreadA before `actionCompletion(ActionCompletionEvent event)` in ThreadB but the ThreadA goes to sleep and when it revives it calls `getActionState()` which calls `activeActions.computeIfAbsent()` which re-adds the actions that was just removed by ThreadB. The other possibility is events are being fired out-of-order.

I wonder if the right fix is for `UiEventHandler` to queue and only fire 1 in-progress event per actionId and wait for each event completion before firing the next. Presumably `computeIfAbsent` is being used because of this exact problem -- events can be processed out-of-order so `actionStarted()` cannot be assumed to have already ran & created an action in `activeActions`.

Patch:
```
$ git diff
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java b/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
index eef7ea5c64..99ae5e36d2 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
@@ -361,6 +361,8 @@ final class UiStateTracker {
 
   private final Map<Artifact, ActionState> activeActions;
 
+  private final Map<Artifact, Boolean> finishedActions;
+
   // running downloads are identified by the original URL they were trying to access.
   private final Deque<String> runningDownloads;
   private final Map<String, Long> downloadNanoStartTimes;
@@ -396,6 +398,7 @@ final class UiStateTracker {
 
   UiStateTracker(Clock clock, int targetWidth) {
     this.activeActions = new ConcurrentHashMap<>();
+    this.finishedActions = new ConcurrentHashMap<>();
 
     this.actionsCompleted = new AtomicInteger();
     this.testActions = new ConcurrentHashMap<>();
@@ -582,6 +585,9 @@ final class UiStateTracker {
     ActionExecutionMetadata action = event.action();
     Artifact actionId = event.action().getPrimaryOutput();
     long now = clock.nanoTime();
+    if (finishedActions.containsKey(actionId)) {
+        System.out.println(String.format("id '%s' is already finished, and event %s is being processed", actionId, event.progress()));
+    }
     getActionState(action, actionId, now).onProgressEvent(event, now);
   }
 
@@ -589,6 +595,7 @@ final class UiStateTracker {
     Action action = event.getAction();
     Artifact actionId = action.getPrimaryOutput();
     checkNotNull(activeActions.remove(actionId), "%s not active after %s", actionId, event);
+    finishedActions.put(actionId, true);
 
     // As callers to the experimental state tracker assume we will fully report the new state once
     // informed of an action completion, we need to make sure the progress receiver is aware of the
@@ -604,6 +611,7 @@ final class UiStateTracker {
     Artifact actionId = action.getPrimaryOutput();
 
     checkNotNull(activeActions.remove(actionId), "%s not active after %s", actionId, event);
+    finishedActions.put(actionId, true);
 
     if (action.getOwner() != null) {
       Label owner = action.getOwner().getLabel();
``` Also for anyone who OK with compiling bazel from source and applying a patch (patch based off `4.2.1`), this fixes it for me:
```
$ git diff
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java b/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
index eef7ea5c64..9e5570e8a7 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
@@ -526,6 +526,11 @@ final class UiStateTracker {
     return activeActions.computeIfAbsent(actionId, (key) -> new ActionState(action, nanoTimeNow));
   }
 
+  @Nullable
+  private ActionState getActionStateIfPresent(Artifact actionId) {
+    return activeActions.get(actionId);
+  }
+
   void actionStarted(ActionStartedEvent event) {
     Action action = event.getAction();
     Artifact actionId = action.getPrimaryOutput();
@@ -582,7 +587,10 @@ final class UiStateTracker {
     ActionExecutionMetadata action = event.action();
     Artifact actionId = event.action().getPrimaryOutput();
     long now = clock.nanoTime();
-    getActionState(action, actionId, now).onProgressEvent(event, now);
+    ActionState actionState = getActionStateIfPresent(actionId);
+    if (actionState != null) {
+        actionState.onProgressEvent(event, now);
+    }
   }
 
   void actionCompletion(ActionScanningCompletedEvent event) {
``` @bjacklyn Would you mind sending your patch as a pull request?
Actions stuck in "active" and thus "[Prepa]"
### Description of the problem:

The UiStateTracker sometimes reports actions in "Preparing" status (shortened to "[Prepa]") indefinitely.
Because this "stuck" action will be the longest running, it will often be the only displayed action (when `--curses=no`).
For example, here is what a sample CI run looks like:
```
[0 / 69] [Prepa] BazelWorkspaceStatusAction stable-status.txt
[15,599 / 17,556] 5 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 13s ... (296 actions, 90 running)
[26,933 / 50,441] 34 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 29s ... (344 actions, 268 running)
[55,044 / 92,565] 89 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 46s ... (438 actions, 275 running)
[57,666 / 93,495] 101 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 66s ... (546 actions, 312 running)
[58,588 / 93,662] 111 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 89s ... (577 actions, 323 running)
[58,915 / 93,691] 111 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 116s ... (577 actions, 324 running)
[59,156 / 93,935] 121 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 147s ... (572 actions, 321 running)
[59,582 / 94,344] 149 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 182s ... (577 actions, 323 running)
[60,425 / 95,053] 168 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 223s ... (578 actions, 325 running)
[61,058 / 95,161] 203 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 270s ... (577 actions, 323 running)
[62,440 / 96,339] 351 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 324s ... (577 actions, 324 running)
[63,079 / 96,913] 420 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 386s ... (578 actions, 324 running)
[64,779 / 114,781] 485 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 457s ... (581 actions, 326 running)
[65,352 / 115,200] 543 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 539s ... (579 actions, 323 running)
[68,085 / 115,383] 589 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 633s ... (581 actions, 323 running)
[76,800 / 115,433] 652 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 741s ... (571 actions, 305 running)
[95,197 / 115,457] 654 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 866s ... (404 actions, 137 running)
[116,032 / 116,315] 828 / 877 tests; [Prepa] Compiling TypeScript (prodmode) //ts/lib:lib; 1009s ... (162 actions, 102 running)
```
Which is just ... not useful.

Note that actual action execution is not affected by this.  This is purely a visual bug.

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

This bug appears to be caused by a race condition, so is not easily reproducible.
I reckon it is a case where an action is added to the `activeActions` map, then removed after it completes, then re-added by a call to `computeIfAbsent` being called in a racy manner.


### What operating system are you running Bazel on?

Linux 20.04

### What's the output of `bazel info release`?

release 4.2.0

I have not been able to reproduce this issue on 4.1.0

src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
src/test/java/com/google/devtools/build/lib/runtime/UiStateTrackerTest.java
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java b/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
index eef7ea5c64ae7c..99f9ce1e0fe69c 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/UiStateTracker.java
@@ -526,6 +526,11 @@ private ActionState getActionState(
     return activeActions.computeIfAbsent(actionId, (key) -> new ActionState(action, nanoTimeNow));
   }
 
+  @Nullable
+  private ActionState getActionStateIfPresent(Artifact actionId) {
+    return activeActions.get(actionId);
+  }
+
   void actionStarted(ActionStartedEvent event) {
     Action action = event.getAction();
     Artifact actionId = action.getPrimaryOutput();
@@ -579,10 +584,12 @@ void runningAction(RunningActionEvent event) {
   }
 
   void actionProgress(ActionProgressEvent event) {
-    ActionExecutionMetadata action = event.action();
     Artifact actionId = event.action().getPrimaryOutput();
     long now = clock.nanoTime();
-    getActionState(action, actionId, now).onProgressEvent(event, now);
+    ActionState actionState = getActionStateIfPresent(actionId);
+    if (actionState != null) {
+      actionState.onProgressEvent(event, now);
+    }
   }
 
   void actionCompletion(ActionScanningCompletedEvent event) {

diff --git a/src/test/java/com/google/devtools/build/lib/runtime/UiStateTrackerTest.java b/src/test/java/com/google/devtools/build/lib/runtime/UiStateTrackerTest.java
index 197ac415e72114..47cf3cc4f50508 100644
--- a/src/test/java/com/google/devtools/build/lib/runtime/UiStateTrackerTest.java
+++ b/src/test/java/com/google/devtools/build/lib/runtime/UiStateTrackerTest.java
@@ -601,6 +601,7 @@ public void actionProgress_visible() throws Exception {
     ManualClock clock = new ManualClock();
     Action action = createDummyAction("Some random action");
     UiStateTracker stateTracker = new UiStateTracker(clock, /* targetWidth= */ 70);
+    stateTracker.actionStarted(new ActionStartedEvent(action, clock.nanoTime()));
     stateTracker.actionProgress(
         ActionProgressEvent.create(action, "action-id", "action progress", false));
     LoggingTerminalWriter terminalWriter = new LoggingTerminalWriter(/*discardHighlight=*/ true);
@@ -619,6 +620,7 @@ public void actionProgress_withTooSmallWidth_progressSkipped() throws Exception
     ManualClock clock = new ManualClock();
     Action action = createDummyAction("Some random action");
     UiStateTracker stateTracker = new UiStateTracker(clock, /* targetWidth= */ 30);
+    stateTracker.actionStarted(new ActionStartedEvent(action, clock.nanoTime()));
     stateTracker.actionProgress(
         ActionProgressEvent.create(action, "action-id", "action progress", false));
     LoggingTerminalWriter terminalWriter = new LoggingTerminalWriter(/*discardHighlight=*/ true);
@@ -637,6 +639,7 @@ public void actionProgress_withSmallWidth_progressShortened() throws Exception {
     ManualClock clock = new ManualClock();
     Action action = createDummyAction("Some random action");
     UiStateTracker stateTracker = new UiStateTracker(clock, /* targetWidth= */ 50);
+    stateTracker.actionStarted(new ActionStartedEvent(action, clock.nanoTime()));
     stateTracker.actionProgress(
         ActionProgressEvent.create(action, "action-id", "action progress", false));
     LoggingTerminalWriter terminalWriter = new LoggingTerminalWriter(/*discardHighlight=*/ true);
@@ -655,6 +658,7 @@ public void actionProgress_multipleProgress_displayInOrder() throws Exception {
     ManualClock clock = new ManualClock();
     Action action = createDummyAction("Some random action");
     UiStateTracker stateTracker = new UiStateTracker(clock, /* targetWidth= */ 70);
+    stateTracker.actionStarted(new ActionStartedEvent(action, clock.nanoTime()));
     stateTracker.actionProgress(
         ActionProgressEvent.create(action, "action-id1", "action progress 1", false));
     stateTracker.actionProgress(
