view: split the deferred FX-park drain by kind — restores stay forced, parks go one FX per idle tick behind a 1s coalescing delay

A rapid A→B→A flip now costs no plugin work: the restore cancels the still-pending
park outright. A park that has already written one FX carries a `partial` flag and is
superseded by its inverse rather than cancelled, so a half-parked chain is never stranded.

tests/test_view_fx_park.cpp

@@ -4,7 +4,9 @@
 // The properties under test: a mode switch leaves its per-FX offline work here,
 // so a second switch arriving before the first drained must leave every track in
 // the state the SECOND switch specifies — never the first's, never both replayed;
-// and a cancel must not strand the pre-park FX state it was the last record of.
+// a cancel must not strand the pre-park FX state it was the last record of; and
+// the restore/park kind split must hold, restores detaching whole while parks
+// stay queued across the ticks that apply them one FX at a time.
 
 #include "../src/shell/view/view_fx_park.h"
 
@@ -233,30 +235,126 @@ static void testRestorePlanOpsRebuildTheSlotKeyedSnapshotVerbatim() {
     CHECK(rebuilt.states == snap.fxOffline);
 }
 
-// -- re-entrancy -------------------------------------------------------------
+// -- the kind split ----------------------------------------------------------
 
-static void testTakeDetachesEverythingAndLeavesTheQueueEmpty() {
+static void testTakeRestoresDetachesRestoresAndLeavesParksQueued() {
+    // The forced drain's slice: a restore is unsafe to persist over and goes now,
+    // a park is safe and stays for the idle tick to work one FX at a time.
     FxParkQueue q;
     q.park("{A}");
     q.restore("{B}", ops("{FX}", true));
+    q.park("{C}");
+    q.restore("{D}", ops("{FX}", false));
 
-    const std::vector<FxParkIntent> taken = q.take();
+    const std::vector<FxParkIntent> taken = q.takeRestores();
 
     CHECK(taken.size() == 2);
-    CHECK(taken.size() == 2 && taken[0].guid == "{A}" && taken[0].park);
-    CHECK(taken.size() == 2 && taken[1].guid == "{B}" && !taken[1].park);
-    CHECK(q.empty());
+    CHECK(taken.size() == 2 && taken[0].guid == "{B}" && !taken[0].park);
+    CHECK(taken.size() == 2 && taken[1].guid == "{D}" && !taken[1].park);
+    // Enqueue order is apply order on BOTH sides of the slice.
+    CHECK(q.pending().size() == 2);
+    CHECK(q.pending().size() == 2 && q.pending()[0].guid == "{A}" && q.pending()[0].park);
+    CHECK(q.pending().size() == 2 && q.pending()[1].guid == "{C}" && q.pending()[1].park);
 }
 
+static void testTakeRestoresWithOnlyParksQueuedTakesNothing() {
+    FxParkQueue q;
+    q.park("{A}");
+
+    CHECK(q.takeRestores().empty());
+    CHECK(q.pending().size() == 1);  // the park is not this drain's to consume
+}
+
+static void testNextParkGuidWalksParksInEnqueueOrderAndSkipsRestores() {
+    FxParkQueue q;
+    q.restore("{R}", ops("{FX}", false));
+    q.park("{A}");
+    q.park("{B}");
+
+    // A PEEK: repeated reads answer the same until the park is retired, and a
+    // restore is never handed to the park tick.
+    CHECK(q.nextParkGuid() == "{A}");
+    CHECK(q.nextParkGuid() == "{A}");
+    q.finishPark("{A}");
+    CHECK(q.nextParkGuid() == "{B}");
+    q.finishPark("{B}");
+    CHECK(q.nextParkGuid().empty());
+    CHECK(q.pending().size() == 1);  // park progress never consumed the restore
+    CHECK(intentFor(q, "{R}") != nullptr);
+}
+
+static void testFinishParkNeverRetiresARestoreStandingAtThatGuid() {
+    FxParkQueue q;
+    q.park("{A}");
+    q.markPartial("{A}");
+    q.restore("{A}", ops("{FX}", false));
+
+    q.finishPark("{A}");
+
+    CHECK(q.pending().size() == 1);
+    CHECK(intentFor(q, "{A}") && !intentFor(q, "{A}")->park);
+}
+
+// -- the lazy park's cancel semantics ----------------------------------------
+
+static void testAnUnstartedParkCancelledByItsRestoreCostsZeroWork() {
+    // The headline property of deferring the park: A→B parks {A}, B→A restores it
+    // before the coalescing delay let one FX move, and the flip costs no plugin
+    // load or unload at all — neither half of the drain has anything left to do.
+    FxParkQueue q;
+    q.park("{A}");
+    q.restore("{A}", ops("{FX}", false));
+
+    CHECK(q.empty());
+    CHECK(q.nextParkGuid().empty());
+    CHECK(q.takeRestores().empty());
+}
+
+static void testAParkThatAlreadyWroteIsSupersededByItsRestoreNotCancelled() {
+    // One FX is offline, so the chain matches NEITHER endpoint. Annihilating here
+    // would leave it offline with the ops that describe its prior state dropped.
+    FxParkQueue q;
+    q.park("{A}");
+    CHECK(q.nextParkGuid() == "{A}");
+    q.markPartial("{A}");  // the tick is about to write this track's first FX
+
+    q.restore("{A}", ops("{FX}", false));
+
+    CHECK(q.pending().size() == 1);
+    const FxParkIntent* held = intentFor(q, "{A}");
+    CHECK(held && !held->park);
+    CHECK(held && held->restoreOps.size() == 1 && held->restoreOps.front().fxGuid == "{FX}");
+    CHECK(q.nextParkGuid().empty());  // no park work left — the restore owns the chain
+}
+
+static void testAParkOnASupersededRestoreResumesRatherThanAnnihilates() {
+    // The mirror hazard: the restore also never ran, so the chain is still half
+    // parked. Cancelling outright would strand every FX the first pass had not
+    // reached yet — the park must resume, and still hand back the pre-park ops.
+    FxParkQueue q;
+    q.park("{A}");
+    q.markPartial("{A}");
+    q.restore("{A}", ops("{FX}", false));
+
+    const std::vector<FxOfflineOp> cancelled = q.park("{A}");
+
+    CHECK(cancelled.size() == 1);
+    CHECK(cancelled.size() == 1 && cancelled.front().fxGuid == "{FX}");
+    CHECK(q.nextParkGuid() == "{A}");
+    CHECK(intentFor(q, "{A}") && intentFor(q, "{A}")->park);
+}
+
+// -- re-entrancy -------------------------------------------------------------
+
 static void testIntentsArrivingDuringADrainSurviveIt() {
     // [verify — DAW] applying an intent loads/unloads plugins, which is ASSUMED to
     // pump the message loop, so a switch can re-enter and enqueue mid-drain. Those
     // intents belong to the NEXT drain — the one in progress must neither see them
     // nor discard them.
     FxParkQueue q;
-    q.park("{A}");
+    q.restore("{A}", ops("{FX}", true));
 
-    const std::vector<FxParkIntent> draining = q.take();
+    const std::vector<FxParkIntent> draining = q.takeRestores();
     q.restore("{B}", ops("{FX}", false));  // arrives while {A} is being applied
 
     CHECK(draining.size() == 1);
@@ -271,7 +369,7 @@ static void testAReEntrantParkCancelsOnlyWhatIsStillPending() {
     // silently annihilate against an intent that has already been applied.
     FxParkQueue q;
     q.restore("{A}", ops("{FX}", false));
-    q.take();
+    q.takeRestores();
 
     const std::vector<FxOfflineOp> cancelled = q.park("{A}");
 
@@ -297,7 +395,13 @@ int main() {
     testSlotKeyedRestoreDoesNotBecomeIdentityKeyedWithNoIdentities();
     testRestorePlanOpsRebuildTheIdentityKeyedSnapshotVerbatim();
     testRestorePlanOpsRebuildTheSlotKeyedSnapshotVerbatim();
-    testTakeDetachesEverythingAndLeavesTheQueueEmpty();
+    testTakeRestoresDetachesRestoresAndLeavesParksQueued();
+    testTakeRestoresWithOnlyParksQueuedTakesNothing();
+    testNextParkGuidWalksParksInEnqueueOrderAndSkipsRestores();
+    testFinishParkNeverRetiresARestoreStandingAtThatGuid();
+    testAnUnstartedParkCancelledByItsRestoreCostsZeroWork();
+    testAParkThatAlreadyWroteIsSupersededByItsRestoreNotCancelled();
+    testAParkOnASupersededRestoreResumesRatherThanAnnihilates();
     testIntentsArrivingDuringADrainSurviveIt();
     testAReEntrantParkCancelsOnlyWhatIsStillPending();