Give a remote discard the time an accept gets

src/DiffEngineTray.Tests/OwnedInlineHostTest.cs

@@ -315,7 +315,7 @@ public async Task DiscardAllEmptiesTheQueue()
         owner.Queue();
         owner.Queue(@"c:\repo\OtherTests.cs", 7);
 
-        owner.Host.DiscardAll();
+        owner.Host.DiscardAll(out _);
 
         await Assert.That(owner.Host.List()).IsEmpty();
     }

src/DiffEngineTray.Tests/StubInlineHost.cs

@@ -59,8 +59,10 @@ public bool AcceptAll(out string? message)
         return AcceptAllSucceeds;
     }
 
-    public void DiscardAll()
+    public bool DiscardAll(out string? message)
     {
+        message = null;
+        return true;
     }
 
     public void Focus(PendingSnapshot snapshot)

src/DiffEngineTray/IInlineHost.cs

@@ -26,7 +26,11 @@ interface IInlineHost
 
     bool Discard(PendingSnapshot snapshot, out string? message);
     bool AcceptAll(out string? message);
-    void DiscardAll();
+    /// <summary>
+    /// False when the queue owner could not be asked, so a caller clearing its own state knows not
+    /// to.
+    /// </summary>
+    bool DiscardAll(out string? message);
 
     /// <summary>
     /// Bring the window forward on this item, launching one if there is none.

src/DiffEngineTray/OwnedInlineHost.cs

@@ -143,8 +143,12 @@ public bool AcceptAll(out string? message)
         }
     }
 
-    public void DiscardAll() =>
-        ((IQueueOwner) this).DiscardAll();
+    public bool DiscardAll(out string? message)
+    {
+        message = ((IQueueOwner) this).DiscardAll();
+        // Owned in this process, so there is nobody to fail to reach
+        return true;
+    }
 
     public void Focus(PendingSnapshot snapshot) =>
         Show(WindowCommand.Focus, snapshot.Key);

src/DiffEngineTray/RemoteInlineHost.cs

@@ -96,8 +96,14 @@ public AcceptOutcome Accept(PendingSnapshot snapshot, out string? message)
             : AcceptOutcome.Applied;
     }
 
+    /// <summary>
+    /// On <see cref="acceptWait"/>, not the short timeout. Discarding is not a clock driven call
+    /// - it comes from the menu or a hot key - and the owner answering it may be busy inside
+    /// InlineApplier, which waits up to ten seconds on its cross process mutex. Half a second
+    /// turned a busy owner into "The snapshot viewer is not running."
+    /// </summary>
     public bool Discard(PendingSnapshot snapshot, out string? message) =>
-        Send(ViewerVerb.Discard, snapshot.Key, ViewerClient.ShortTimeout, out message);
+        Send(ViewerVerb.Discard, snapshot.Key, acceptWait, out message);
 
     /// <summary>
     /// True only when the queue is empty afterwards, for the reason <see cref="Accept"/> gives —
@@ -110,8 +116,14 @@ public bool AcceptAll(out string? message) =>
         Send(ViewerVerb.AcceptAll, null, acceptWait, out message) &&
         List().Count == 0;
 
-    public void DiscardAll() =>
-        Send(ViewerVerb.DiscardAll, null, ViewerClient.ShortTimeout, out _);
+    /// <summary>
+    /// As <see cref="Discard"/>, and the outcome is returned rather than dropped. Discarded on a
+    /// busy owner used to do nothing at all while Tracker.Clear went ahead and emptied its own
+    /// snapshot list, so "Discard (n)" reported success and everything reappeared on the next
+    /// scan two seconds later.
+    /// </summary>
+    public bool DiscardAll(out string? message) =>
+        Send(ViewerVerb.DiscardAll, null, acceptWait, out message);
 
     public void Focus(PendingSnapshot snapshot) =>
         Send(ViewerVerb.Focus, snapshot.Key, ViewerClient.ShortTimeout, out _);

src/DiffEngineTray/Tracker.cs

@@ -671,8 +671,17 @@ public void Clear()
     {
         ((ITrackedFiles) this).DiscardAll();
 
-        inline.DiscardAll();
-        snapshots = [];
+        // Only forget the cached snapshots when the owner actually discarded them. It used to be
+        // cleared regardless, so a discard the owner never received still emptied the menu - and
+        // everything came back on the next scan two seconds later
+        if (inline.DiscardAll(out var message))
+        {
+            snapshots = [];
+        }
+        else
+        {
+            Log.Error(message ?? "Could not discard the pending snapshots.");
+        }
     }
 
     /// <summary>