feat(panel): add Insert as FX — ReaSampler 9000 onto the selected track, preloaded with the focused capture, over the existing instrument-drop body

src/app/CMakeLists.txt

@@ -52,9 +52,10 @@ add_library(reaper_reasampler MODULE
     ${REASAMPLER_SRC_DIR}/shell/actions/arrange_drop_win.cpp
     ${REASAMPLER_SRC_DIR}/shell/actions/drag_out_win.cpp
     ${REASAMPLER_SRC_DIR}/shell/actions/instrument_drop_win.cpp
+    ${REASAMPLER_SRC_DIR}/shell/actions/insert_fx_action.cpp
     ${REASAMPLER_SRC_DIR}/shell/persist/usage_scan.cpp
 )
-target_link_libraries(reaper_reasampler PRIVATE json wire file_bytes bank_model capture_paths capture_name peaks bank_grid mode_switch tab_strip view_mode_model view_tree guid_diff lane_keys solo_cache insert_plan render_settings render_window track_topology batch_capture tail_control capture_realtime bank_book wav_codec origin_ledger tracking_authority prune_reconcile prune_button app_version provenance drag_out instrument_drop theme component_geometry action_bar footer_bar overflow_menu mode_enable tooltip card_meta card_drag assignment_request bank_sync sample_usage bake_wire resample_name export_bank package_pickers)
+target_link_libraries(reaper_reasampler PRIVATE json wire file_bytes bank_model capture_paths capture_name peaks bank_grid mode_switch tab_strip view_mode_model view_tree guid_diff lane_keys solo_cache insert_plan render_settings render_window track_topology batch_capture tail_control capture_realtime bank_book wav_codec origin_ledger tracking_authority prune_reconcile prune_button app_version provenance drag_out instrument_drop theme component_geometry action_bar footer_bar overflow_menu mode_enable insert_fx_enable tooltip card_meta card_drag assignment_request bank_sync sample_usage bake_wire resample_name export_bank package_pickers)
 # NOT linked here, deliberately: sampler_core / pitch_shift / the filter / limiter. The
 # instrument renders its own bake in its own process, which is what keeps the extension's
 # link graph free of the voice engine — a link edge to it here means the design drifted.

src/app/main.cpp

@@ -26,6 +26,7 @@
 #include "shell/actions/action_registry.h"       // the registration table
 #include "shell/actions/bank_actions.h"          // multi-bank action family
 #include "shell/actions/design_view_actions.h"   // Design View action family
+#include "shell/actions/insert_fx_action.h"      // insert-as-FX action body
 #include "shell/actions/package_export_action.h" // bank-package export action body
 #include "shell/actions/package_import_action.h" // bank-package import action body
 #include "core/wire/bake_wire.h"                 // kBakeActionSuffix (the shared action id)
@@ -87,6 +88,9 @@ static void RunCaptureItemAssign(int) { capture::RunCaptureItemAssign(g_session)
 static void RunInsertSelected(int arg) {
     capture::RunInsertSelected(g_session, arg != 0);
 }
+// Placement of the PLAYER, not of an item: adds a ReaSampler 9000 to the selected track's
+// FX chain. Sessionless — it reads the panel's selection, exactly like the drag it mirrors.
+static void RunInsertAsFx(int)        { reasampler::doInsertAsFx(); }
 static void RunBatchCaptureItems(int) { capture::RunBatchCaptureItems(g_session); }
 static void RunBatchCaptureRazor(int) { capture::RunBatchCaptureRazor(g_session); }
 static void RunCaptureRealtime(int)   { capture::RunCaptureRealtimeTrack(g_session); }
@@ -126,6 +130,9 @@ static std::vector<reasampler::ActionTableRow> buildMainActionTable() {
     rows.push_back({"INSERT_SELECTED_CONFORM",
                     "insert selected sample at edit cursor (conform to tempo)",
                     &RunInsertSelected, 1});
+    rows.push_back({"INSERT_AS_FX",
+                    "insert selected sample as ReaSampler 9000 on the selected track",
+                    &RunInsertAsFx});
     // One action fires N captures (per selected item / per razor area); the original
     // selection is restored on every exit path. Bank-only, never places.
     rows.push_back({"CAPTURE_BATCH_ITEMS",

src/core/ui/CLAUDE.md

@@ -104,6 +104,7 @@ L7 sub-pass, 2026-07-27):
 - `footer_bar` — pure footer layout/hit-test: `[Arrange|Design]` mode-toggle geometry, Tail button, and Prune placement, plus `modeSegmentEnabled` — the mode segment's live/dead predicate under the playback gate AND under whether the shell resolved a routable command id for it (both bools passed IN, so this stays REAPER-free).
 - `overflow_menu` — pure overflow-menu-button geometry/reserve/hit-test for the top-toolbar More (⋯) button.
 - `mode_enable` — pure opposite-mode enablement predicate: given the active mode, computes per-button live/disabled state for the four Item/Track × Arrange/Design tag buttons.
+- `insert_fx_enable` — pure refusal fold behind the Insert-as-FX verb (one instance holds one capture, so it needs exactly one focused capture AND a destination track), plus the button's live/dead bit derived from that same fold and one sentence per refusal. The payload/destination axes are ordered deliberately because they are knowable at different times — the header states which half the button may speak for.
 - `tooltip` — pure tooltip placement + prefix-strip: strips the `ReaSampler:` display prefix from the registered action phrase; width clamped to the client rect.
 - `card_drag` — pure drag-gesture precedence + slot hit-test: leave-client → OS drag-out; other-bank → move/copy; same-bank → reorder / Alt-over-occupied → replace.
 - `card_meta` — pure card-metadata formatters: bars.beats.subdivisions and seconds.milliseconds; blank when the sample is unstamped. Also `cardNameStrip` + the two strip constants — the card's name line sits across the TOP of the cell, drawn over the waveform exactly as the length read-out is over it at the bottom, and is suppressed entirely on a cell with no room for both strips plus a waveform band.

src/core/ui/CMakeLists.txt

@@ -17,7 +17,12 @@ reasampler_pure_library(component_geometry SOURCES component_geometry.cpp)
 reasampler_test(component_geometry LINK component_geometry)
 
 reasampler_pure_library(action_bar SOURCES action_bar.cpp)
-reasampler_test(action_bar LINK action_bar)
+# overflow_menu supplies the More-button reserve the panel subtracts before tiling; the
+# bar's narrow-width behaviour is only meaningful against that composition.
+reasampler_test(action_bar LINK action_bar overflow_menu)
+
+reasampler_pure_library(insert_fx_enable SOURCES insert_fx_enable.cpp)
+reasampler_test(insert_fx_enable LINK insert_fx_enable)
 
 # prune_button owns the FooterRect input type the footer layout reuses.
 reasampler_pure_library(footer_bar SOURCES footer_bar.cpp LINK PUBLIC prune_button)

src/core/ui/insert_fx_enable.cpp

@@ -0,0 +1,36 @@
+// insert_fx_enable — pure implementation. See insert_fx_enable.h.
+
+#include "core/ui/insert_fx_enable.h"
+
+namespace reasampler::ui {
+
+InsertFxRefusal insertFxRefusal(int focusedCaptureCount, bool trackSelected) {
+    if (focusedCaptureCount <= 0) return InsertFxRefusal::NoCapture;
+    if (focusedCaptureCount > 1) return InsertFxRefusal::MultiCapture;
+    if (!trackSelected) return InsertFxRefusal::NoTrack;
+    return InsertFxRefusal::None;
+}
+
+bool insertFxButtonEnabled(int focusedCaptureCount) {
+    return insertFxRefusal(focusedCaptureCount, /*trackSelected=*/true) ==
+           InsertFxRefusal::None;
+}
+
+std::string insertFxRefusalMessage(InsertFxRefusal refusal) {
+    switch (refusal) {
+        case InsertFxRefusal::None:
+            return {};
+        case InsertFxRefusal::NoCapture:
+            return "select a capture in the bank panel first, then insert it as an "
+                   "instrument on the selected track";
+        case InsertFxRefusal::MultiCapture:
+            return "select exactly ONE capture -- a ReaSampler 9000 instance holds one "
+                   "capture, so a multi-capture selection has no single sound to load";
+        case InsertFxRefusal::NoTrack:
+            return "select a track first -- the instrument is added to the selected "
+                   "track's FX chain";
+    }
+    return {};
+}
+
+} // namespace reasampler::ui

src/core/ui/insert_fx_enable.h

@@ -0,0 +1,33 @@
+#pragma once
+// insert_fx_enable — refusal fold behind the Insert-as-FX verb (bank_panel Placement
+// button + its bindable action). One ReaSampler 9000 instance holds exactly one capture,
+// so the verb needs exactly one focused capture AND a destination track.
+
+#include <string>
+
+namespace reasampler::ui {
+
+// Why the verb cannot run, or None. The two axes are deliberately ordered payload-first,
+// because they are knowable at DIFFERENT times: the panel selection is authoritative at
+// draw time, so the button paints dead for it; track selection lives in the project with
+// no change callback, so it is only ever spoken as a message at press time. A live button
+// therefore does NOT imply NoTrack was ruled out — every caller must run the full fold.
+enum class InsertFxRefusal {
+    None,
+    NoCapture,
+    MultiCapture,
+    NoTrack,
+};
+
+InsertFxRefusal insertFxRefusal(int focusedCaptureCount, bool trackSelected);
+
+// The button's live/dead bit: the payload axis alone, derived from the same fold so the
+// drawn state and the pressed outcome cannot disagree about the capture rules.
+bool insertFxButtonEnabled(int focusedCaptureCount);
+
+// One sentence per refusal, prefix- and punctuation-free so the shell can frame it. The
+// wording lives here rather than in the shell so "no silent refusal" is assertable
+// without a DAW. None yields an empty string — nothing to say about a verb that ran.
+std::string insertFxRefusalMessage(InsertFxRefusal refusal);
+
+} // namespace reasampler::ui

src/shell/actions/CLAUDE.md

@@ -43,6 +43,7 @@ is owned by other directories and only skinned here.
 - `package_export_action` — the "export bank as package" skin: survey and report first, confirm what is absent (and, separately, a destination being replaced), pick a destination, write. Every prompt in the flow lives here so `shell/package/export_bank` stays promptless. Read-only against the project — it holds the session by `const&`, so no ext-state write, generation bump or undo point is reachable. Registration rides `main.cpp`'s action table (`EXPORT_BANK_PACKAGE`); the panel's tab menu is the second skin over the same body.
 - `drag_out_win` — OS drag-out shell: Windows OLE `DoDragDrop`/`CF_HDROP`, copy-only (`DROPEFFECT_MOVE` not offered); macOS/Linux via `SWELL_InitiateDragDropOfFileList`.
 - `instrument_drop_win` — instrument-drop shell: `probeDropTarget` resolves a screen point to a track + a `ReaperSurface` (via the pure `wire::classifyReaperSurface`, whose token rules `core/wire/CLAUDE.md` owns), and the drop half adds a ReaSampler 9000 instance and applies the dragged capture's state via a transient `.vstpreset` + `TrackFX_SetPreset` (the former `TrackFX_SetNamedConfigParm` "vst_chunk" write was silently unappliable for VST3). Exposes `loadInstrumentOntoTrack` (inner half, no own undo block) and `performInstrumentDrop` (wraps in its own undo block). **Never captures, never writes the bank, never inserts a timeline item.**
+- `insert_fx_action` — the "Insert as FX" verb: a second ENTRY POINT to `instrument_drop_win`'s drop body (the bank panel's Placement button and a bindable action), loading the panel's focused capture onto the first selected track. Owns no insert logic of its own — it resolves the selection, folds `core/ui/insert_fx_enable`, and calls `performInstrumentDrop` (which keeps the one undo block). Refuses visibly on every failing precondition; the panel button additionally paints dead for the two panel-local ones.
 - `arrange_drop_win` — the drag-out gesture's arrange outcome: `arrangeTimeAtScreenX` (pointer column → time via `GetSet_ArrangeView2`'s one-pixel-span reading — inferred, not SDK-documented) and `performArrangeDrop` (snap the drop time, then one `InsertMedia` per capture on the pointer's track — assumed, not confirmed, to land end-to-end via REAPER's own cursor advance — in ONE undo block, counting only InsertMedia's reported successes, with the caller's track selection and edit cursor restored). The one timeline-placing shell here, per the invariant above; it never captures and never writes the bank.
 - `package_import_action` — the bindable / bank-menu / file-drop skin over `shell/package`'s `importBankPackage`. Owns the **ledger gate**, which runs BEFORE the file picker (a refusal must not cost the user a file choice) and is keyed on the session's `LedgerStatus` alone — never on `PruneReport::blockedByTracking`, whose undecodable-`rsusage_*` arm governs deletion-time protection and would refuse an import that only writes birth records. Builds and shows every message the import produces, but the ledger-refusal body itself is `core/package::ledgerRefusalMessage` — a pure fold this TU only supplies the channel-correct namespace to — so the wording is assertable without a DAW. `doImportBankPackage`/`doImportBankPackageFile` return the minted bank id on a landed import (empty otherwise) so a caller can focus it; the verb itself is promptless.
 - `ingest` — ingest-through-the-bank shell on the EXTENSION side: three surfaces — (1) arrange capture→bank→assign (bindable action), (2) Media-Explorer import→bank→instrument on the selected track, (3) file drop onto the bank panel→bank only. Only surface (1) writes the `assignment_request` ext-state wire. **ingest NEVER inserts a timeline item.** Surface (2)'s action is the one in this directory published into a NON-main action section (Media Explorer) as well as Main — two ids, one handler, two dispatch hooks; see root `CLAUDE.md` §"REAPER extension contract" for the mechanism.

src/shell/actions/insert_fx_action.cpp

@@ -0,0 +1,54 @@
+// insert_fx_action.cpp — see insert_fx_action.h. main.cpp owns the API pointers; this TU
+// gets them extern via the WANT list.
+
+#include "shell/actions/insert_fx_action.h"
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+#include "core/ui/insert_fx_enable.h"           // the refusal fold + its wording
+#include "core/wire/instrument_drop.h"          // buildInstrumentDropPreset — the pure payload
+#include "shell/actions/instrument_drop_win.h"  // performInstrumentDrop — the shared drop body
+#include "shell/panel/panel_bank_ops.h"         // bankPanelSelectedSampleIds
+
+#include "reaper_plugin.h"
+
+#define REAPERAPI_MINIMAL
+#define REAPERAPI_WANT_GetSelectedTrack
+#define REAPERAPI_WANT_ShowConsoleMsg
+#include "reaper_plugin_functions.h"
+
+namespace reasampler {
+
+namespace {
+
+void report(const std::string& sentence) {
+    if (ShowConsoleMsg) ShowConsoleMsg(("ReaSampler: " + sentence + "\n").c_str());
+}
+
+}  // namespace
+
+void doInsertAsFx() {
+    const std::vector<std::string> selected = bankPanelSelectedSampleIds();
+    // seltrackidx 0 = the FIRST selected track, master excluded (SDK header) — the same
+    // "the selected track" the Media-Explorer ingest surface targets. A multi-track
+    // selection therefore loads onto the first, rather than refusing or fanning out.
+    MediaTrack* track = GetSelectedTrack ? GetSelectedTrack(nullptr, 0) : nullptr;
+
+    const ui::InsertFxRefusal refusal =
+        ui::insertFxRefusal(static_cast<int>(selected.size()), track != nullptr);
+    if (refusal != ui::InsertFxRefusal::None) {
+        report(ui::insertFxRefusalMessage(refusal));
+        return;
+    }
+
+    // performInstrumentDrop opens its own undo block (one Ctrl-Z) and rolls the FX back
+    // itself if the preset never applies — there is nothing here to group with it, so no
+    // second block is opened around it.
+    if (!performInstrumentDrop(track, wire::buildInstrumentDropPreset(selected.front())))
+        report("could not add ReaSampler 9000 to the selected track -- check the plug-in "
+               "is installed and scanned");
+}
+
+}  // namespace reasampler

src/shell/actions/insert_fx_action.h

@@ -0,0 +1,14 @@
+#pragma once
+// insert_fx_action — the "Insert as FX" verb: a ReaSampler 9000 instance on the SELECTED
+// track, preloaded with the bank panel's focused capture. A second ENTRY POINT to
+// instrument_drop_win's drop body — reached by a button or a keybinding instead of a
+// drag — never a second insert path, so that body's undo block and its
+// never-capture/never-place guarantees carry over unchanged.
+
+namespace reasampler {
+
+// Refuses visibly on every failing precondition (no capture / many captures / no track):
+// see core/ui/insert_fx_enable for the fold and its wording.
+void doInsertAsFx();
+
+}  // namespace reasampler

src/shell/panel/CLAUDE.md

@@ -10,8 +10,8 @@ VST3 editor (`shell/instrument/`).
 
 Pure layout/hit-test/palette modules the panel draws through (`theme`,
 `component_geometry`, `bank_grid`, `tab_strip`, `mode_switch`, `action_bar`,
-`footer_bar`, `overflow_menu`, `prune_button`, `mode_enable`, `tooltip`, `card_drag`,
+`footer_bar`, `overflow_menu`, `prune_button`, `mode_enable`, `insert_fx_enable`,
-`card_meta`) live in `core/ui` / `core/model` and are documented there — this
+`tooltip`, `card_drag`, `card_meta`) live in `core/ui` / `core/model` and are documented there — this
 directory consumes them but does not own them. The promptless bank-mutation verbs
 (`bankOpCreate`/`Rename`/`Delete`/… + `persistBankOp`) that `panel_bank_ops` skins
 live in `shell/bank_ops`, a sibling directory, not here.

src/shell/panel/panel_layout.cpp

@@ -128,6 +128,14 @@ std::vector<ActionBarRow> topBarRows() {
     rows.push_back({"INSERT_SELECTED_CONFORM", "Insert Conform",
                     "insert selected sample at edit cursor (conform to tempo)",
                     ActionCluster::Placement, true});
+    // The one top-bar row with a live/dead gate: it places the PLAYER, and one instance
+    // holds one capture. Only the payload half of the fold is knowable at draw time —
+    // core/ui/insert_fx_enable.h states why the track half stays a press-time message.
+    rows.push_back({"INSERT_AS_FX", "Insert as FX",
+                    "insert selected sample as ReaSampler 9000 on the selected track",
+                    ActionCluster::Placement,
+                    insertFxButtonEnabled(
+                        static_cast<int>(g_panel.selection.indices.size()))});
     return rows;
 }
 

src/shell/panel/panel_state.h

@@ -41,6 +41,7 @@
 #include "core/ui/component_geometry.h"
 #include "core/ui/drag_out.h"
 #include "core/ui/footer_bar.h"
+#include "core/ui/insert_fx_enable.h"
 #include "core/ui/mode_enable.h"
 #include "core/ui/overflow_menu.h"
 #include "core/ui/prune_button.h"
@@ -133,6 +134,7 @@ using ui::hitTestMenuButton;
 using ui::hitTestPruneButton;
 using ui::hitTestSlot;
 using ui::hitTestTabStrip;
+using ui::insertFxButtonEnabled;
 using ui::kCardNameScrimAlpha;
 using ui::kCardStripHeight;
 using ui::kCardStripPad;
@@ -401,8 +403,9 @@ struct ActionBarRow {
     std::string shortLabel;
     std::string fullName;
     ActionCluster cluster = ActionCluster::Capture;
-    bool enabled = true;  // opposite-mode gate for bottom-bar tag buttons; always true
+    bool enabled = true;  // opposite-mode gate for the bottom-bar tag buttons, and the
-                          // for the top bar (unconditional triggers)
+                          // payload gate for the top bar's Insert as FX; every other top-
+                          // bar row is an unconditional trigger and stays true
 };
 
 // Modifier state at event time. Alt = the replace modifier.

tests/test_action_bar.cpp

@@ -6,7 +6,8 @@
 //   * Layout: correct rects for each action button across representative panel widths; buttons
 //     pack at a fixed width with intra-cluster + inter-cluster gaps.
 //   * Overflow/hiding when the bar is too narrow (whole trailing buttons dropped, never
-//     clipped; earlier frequent clusters survive).
+//     clipped; earlier frequent clusters survive), and the More-button reserve the panel
+//     subtracts before tiling (composed here with overflow_menu, as panel_layout does).
 //   * Label sub-rect correct — spans full button height (L6: keybinding sub-row removed from
 //     the face; binding is in the hover tooltip instead).
 //   * Task grouping reflected STRUCTURALLY: each slot carries its cluster; the flat index runs
@@ -16,6 +17,7 @@
 //   * Resize: no inventory item cut off or overlapping across a representative width range.
 
 #include "../src/core/ui/action_bar.h"
+#include "../src/core/ui/overflow_menu.h"
 
 #include <cstddef>
 #include <cstdio>
@@ -28,14 +30,15 @@ static int g_fail = 0;
 #define CHECK(cond) do { if(!(cond)) { \
     std::printf("FAIL line %d: %s\n", __LINE__, #cond); ++g_fail; } } while(0)
 
-// The panel's real inventory shape (L6): 2 capture, 1 maintenance (Re-capture), 2 placement = 5
+// The panel's real inventory shape: 2 capture, 1 maintenance (Re-capture), 3 placement
-// buttons. Cluster order: Capture -> Maintenance -> Placement (Re-capture sits between the two
+// (Insert / Insert Conform / Insert as FX) = 6 buttons. Cluster order:
-// capture verbs and the placement verbs — the L6 bar ordering).
+// Capture -> Maintenance -> Placement (Re-capture sits between the two capture verbs and the
+// placement verbs).
 static std::vector<ClusterSpec> inventory() {
     return {
         {ActionCluster::Capture, 2},
         {ActionCluster::Maintenance, 1},
-        {ActionCluster::Placement, 2},
+        {ActionCluster::Placement, 3},
     };
 }
 
@@ -52,7 +55,7 @@ static ActionBarSpec roundSpec() {
 
 // --- Layout: all fit, correct rects + gaps ------------------------------------
 
-// A wide bar fits all 5 buttons. Verify the first few rects, the intra-cluster gap, and the
+// A wide bar fits all 6 buttons. Verify the first few rects, the intra-cluster gap, and the
 // (wider) inter-cluster gap between button 1 (last capture) and button 2 (maintenance).
 //
 // Pixel walk with roundSpec and bar at (0,40):
@@ -61,18 +64,19 @@ static ActionBarSpec roundSpec() {
 //   btn2 (Maintenance): x=228, right=328   (clusterGap = 16 after btn1's right)
 //   btn3 (Placement):   x=344, right=444   (clusterGap = 16 after btn2's right)
 //   btn4 (Placement):   x=448, right=548   (intraGap = 4 after btn3's right)
-//   Minimum bar width: 548 + sidePad(8) = 556. Give it 600.
+//   btn5 (Placement):   x=552, right=652   (intraGap = 4 after btn4's right)
+//   Minimum bar width: 652 + sidePad(8) = 660. Give it 700.
 static void testAllFitRectsAndGaps() {
     const auto clusters = inventory();
     const ActionBarSpec spec = roundSpec();
-    ActionBarRect bar{0, 40, 600, 34};
+    ActionBarRect bar{0, 40, 700, 34};
 
     BarFit fit = computeBarFit(bar, clusters, spec);
-    CHECK(fit.visibleCount == 5);
+    CHECK(fit.visibleCount == 6);
     CHECK(fit.hiddenCount == 0);
 
     auto slots = computeBarSlots(bar, clusters, spec);
-    CHECK(slots.size() == 5);
+    CHECK(slots.size() == 6);
 
     // Button 0: at sidePad, top = y + verticalInset, height = barH - 2*inset.
     CHECK(slots[0].x == 8);
@@ -101,6 +105,13 @@ static void testAllFitRectsAndGaps() {
     CHECK(slots[4].cluster == ActionCluster::Placement);
     CHECK(slots[4].index == 4);
 
+    // Button 5 (Insert as FX — third placement): intra-cluster gap of 4 after 548 -> 552.
+    // It joins the Placement cluster, so no cluster gap opens before it.
+    CHECK(slots[5].x == 552);
+    CHECK(slots[5].cluster == ActionCluster::Placement);
+    CHECK(slots[5].index == 5);
+    CHECK(slots[5].x - (slots[4].x + slots[4].width) == spec.buttonGap);
+
     // Inter-cluster gap (slot[2].x - slot[1].right = 228 - 212 = 16) is wider than the
     // intra-cluster gap (slot[1].x - slot[0].right = 112 - 108 = 4) — task grouping is
     // structurally visible in the geometry.
@@ -155,7 +166,7 @@ static void testOverflowDropsTrailingWhole() {
     ActionBarRect bar{0, 0, 220, 34};
     BarFit fit = computeBarFit(bar, clusters, spec);
     CHECK(fit.visibleCount == 2);
-    CHECK(fit.hiddenCount == 3);
+    CHECK(fit.hiddenCount == 4);
 
     auto slots = computeBarSlots(bar, clusters, spec);
     CHECK(slots.size() == 2);
@@ -175,10 +186,49 @@ static void testTooNarrowForAny() {
     ActionBarRect bar{0, 0, 60, 34};        // sidePad*2 + one 100-wide button won't fit
     BarFit fit = computeBarFit(bar, clusters, spec);
     CHECK(fit.visibleCount == 0);
-    CHECK(fit.hiddenCount == 5);
+    CHECK(fit.hiddenCount == 6);
     CHECK(computeBarSlots(bar, clusters, spec).empty());
 }
 
+// --- Overflow reserve: the bar never tiles under the More (...) button --------
+
+// The panel hands action_bar the top band MINUS overflow_menu's reserve (panel_layout's
+// topToolbarActionRect). Reproduce that composition and assert the invariant it exists to
+// buy: across every width, no visible button's right edge crosses into the reserved strip
+// where the More button is drawn — including the widths where the reserve is what pushes
+// the trailing "Insert as FX" button into overflow.
+static void testOverflowReserveHonouredAcrossWidths() {
+    const auto clusters = inventory();
+    const ActionBarSpec spec = roundSpec();
+    const MenuButtonSpec menuSpec;  // the panel's kMenuBtnSpec defaults
+
+    bool sawReserveCostAButton = false;
+    for (int w = 40; w <= 900; w += 3) {
+        const MenuBarRect band{0, 0, w, 34};
+        const int reserve = menuButtonReserve(band, menuSpec);
+        CHECK(reserve >= 0);
+
+        ActionBarRect action{0, 0, w - reserve, 34};
+        if (action.width < 0) action.width = 0;
+
+        const MenuButtonRect more = computeMenuButton(band, menuSpec);
+        for (const auto& s : computeBarSlots(action, clusters, spec)) {
+            // Fully clear of the reserved strip at the band's right.
+            CHECK(s.x + s.width <= w - reserve);
+            // And so, transitively, clear of the More button itself when one is drawn.
+            if (!more.empty()) CHECK(s.x + s.width <= more.x);
+        }
+
+        // The reserve genuinely costs buttons somewhere in this range, so the assertions
+        // above are exercised against a bar the reserve actually narrowed.
+        const int full = computeBarFit(ActionBarRect{0, 0, w, 34}, clusters, spec).visibleCount;
+        const int reserved = computeBarFit(action, clusters, spec).visibleCount;
+        CHECK(reserved <= full);
+        if (reserved < full) sawReserveCostAButton = true;
+    }
+    CHECK(sawReserveCostAButton);
+}
+
 // --- Degenerate --------------------------------------------------------------
 
 static void testDegenerate() {
@@ -305,6 +355,7 @@ int main() {
     testLabelFullHeightWhenShort();
     testOverflowDropsTrailingWhole();
     testTooNarrowForAny();
+    testOverflowReserveHonouredAcrossWidths();
     testDegenerate();
     testHitTestHitsButtons();
     testHitTestGapsAreMisses();

tests/test_insert_fx_enable.cpp

@@ -0,0 +1,91 @@
+// Standalone tests for reasampler::ui::insert_fx_enable — no REAPER, no test framework.
+// Exhaustive over the fold's whole input space (track selected x capture-count class),
+// plus the two properties the shell leans on: the button bit is the fold restricted to
+// the payload axis, and every refusal carries a non-empty sentence (a silent refusal is
+// the failure mode this verb must not have).
+
+#include "../src/core/ui/insert_fx_enable.h"
+
+#include <cstdio>
+#include <string>
+
+using namespace reasampler::ui;
+
+static int g_fail = 0;
+#define CHECK(cond) do { if(!(cond)) { \
+    std::printf("FAIL line %d: %s\n", __LINE__, #cond); ++g_fail; } } while(0)
+
+// All six combinations of {no track, track} x {0, 1, many captures}. The payload axis is
+// checked BEFORE the destination axis, so a press with neither a capture nor a track
+// reports the capture problem — the one the user can fix inside the panel they clicked.
+static void testEveryCombination() {
+    CHECK(insertFxRefusal(0, false) == InsertFxRefusal::NoCapture);
+    CHECK(insertFxRefusal(0, true) == InsertFxRefusal::NoCapture);
+    CHECK(insertFxRefusal(1, false) == InsertFxRefusal::NoTrack);
+    CHECK(insertFxRefusal(1, true) == InsertFxRefusal::None);
+    CHECK(insertFxRefusal(3, false) == InsertFxRefusal::MultiCapture);
+    CHECK(insertFxRefusal(3, true) == InsertFxRefusal::MultiCapture);
+}
+
+// Exactly one capture is the only count that can run; two is already too many.
+static void testOnlyASingleCaptureRuns() {
+    CHECK(insertFxRefusal(2, true) == InsertFxRefusal::MultiCapture);
+    for (int n = 0; n <= 8; ++n) {
+        const bool ready = insertFxRefusal(n, true) == InsertFxRefusal::None;
+        CHECK(ready == (n == 1));
+    }
+}
+
+// A negative count can only come from a caller bug; it must fold to a refusal, never to
+// Ready (fail closed — this verb writes to the project).
+static void testNegativeCountRefuses() {
+    CHECK(insertFxRefusal(-1, true) == InsertFxRefusal::NoCapture);
+    CHECK(!insertFxButtonEnabled(-1));
+}
+
+// The button bit is the fold with the destination axis held satisfied: it goes dead for
+// exactly the two panel-local refusals and stays live otherwise, whatever the project's
+// track selection happens to be.
+static void testButtonBitIsThePayloadAxisOfTheFold() {
+    for (int n = -1; n <= 4; ++n) {
+        const bool live = insertFxButtonEnabled(n);
+        CHECK(live == (insertFxRefusal(n, true) == InsertFxRefusal::None));
+        // A dead button always corresponds to a capture-side refusal, never to NoTrack.
+        if (!live) {
+            const InsertFxRefusal r = insertFxRefusal(n, false);
+            CHECK(r == InsertFxRefusal::NoCapture || r == InsertFxRefusal::MultiCapture);
+        }
+    }
+    CHECK(insertFxButtonEnabled(1));
+    CHECK(!insertFxButtonEnabled(0));
+    CHECK(!insertFxButtonEnabled(2));
+}
+
+// Every refusal says something, and each says something DIFFERENT — a shared sentence
+// would leave the user unable to tell which condition they hit. None says nothing.
+static void testEveryRefusalHasItsOwnSentence() {
+    const std::string none = insertFxRefusalMessage(InsertFxRefusal::None);
+    const std::string noCap = insertFxRefusalMessage(InsertFxRefusal::NoCapture);
+    const std::string multi = insertFxRefusalMessage(InsertFxRefusal::MultiCapture);
+    const std::string noTrk = insertFxRefusalMessage(InsertFxRefusal::NoTrack);
+
+    CHECK(none.empty());
+    CHECK(!noCap.empty());
+    CHECK(!multi.empty());
+    CHECK(!noTrk.empty());
+    CHECK(noCap != multi);
+    CHECK(noCap != noTrk);
+    CHECK(multi != noTrk);
+}
+
+int main() {
+    testEveryCombination();
+    testOnlyASingleCaptureRuns();
+    testNegativeCountRefuses();
+    testButtonBitIsThePayloadAxisOfTheFold();
+    testEveryRefusalHasItsOwnSentence();
+
+    if (g_fail == 0) std::printf("insert_fx_enable: all tests passed\n");
+    else std::printf("insert_fx_enable: %d CHECK(s) FAILED\n", g_fail);
+    return g_fail == 0 ? 0 : 1;
+}