diff --git a/src/components/KeySignaturePickerPopup.css b/src/components/KeySignaturePickerPopup.css
new file mode 100644
index 0000000..bd10e65
--- /dev/null
+++ b/src/components/KeySignaturePickerPopup.css
@@ -0,0 +1,184 @@
+.key-signature-popup {
+  min-width: 270px;
+  padding: 12px 12px 10px;
+}
+
+.key-signature-picker {
+  width: 252px;
+  height: 224px;
+  position: relative;
+  color: #e0e0e0;
+  font-family: Georgia, 'Times New Roman', serif;
+}
+
+.key-signature-picker-title {
+  position: absolute;
+  left: 50%;
+  transform: translateX(-50%);
+  margin: 0;
+  color: #f0f0f0;
+  letter-spacing: 0.4px;
+  pointer-events: none;
+  font-weight: 600;
+}
+
+.key-signature-picker-title.major {
+  top: 2px;
+  font-size: 10px;
+}
+
+.key-signature-picker-title.minor {
+  top: 60px;
+  font-size: 9px;
+  color: #9fc89c;
+}
+
+.key-signature-picker-ring {
+  position: absolute;
+  inset: 32px 40px 30px;
+  border-radius: 50%;
+  background:
+    radial-gradient(circle at center, transparent 0 38%, rgba(33, 33, 33, 0.96) 38% 49%, transparent 49%),
+    conic-gradient(
+      from 0deg,
+      rgba(84, 84, 84, 0.28),
+      rgba(69, 69, 69, 0.5),
+      rgba(84, 84, 84, 0.28)
+    );
+  box-shadow: inset 0 0 0 1px rgba(255, 255, 255, 0.06);
+  pointer-events: none;
+}
+
+.key-signature-picker-slot {
+  position: absolute;
+  transform: translate(-50%, -50%);
+}
+
+.key-signature-picker-slot-outer,
+.key-signature-picker-slot-inner,
+.key-signature-picker-slot-count {
+  position: absolute;
+  left: 50%;
+  top: 50%;
+  transform: translate(-50%, -50%);
+}
+
+.key-signature-picker-slot-count {
+  font-size: 7px;
+  color: #b8b8b8;
+  white-space: nowrap;
+  pointer-events: none;
+}
+
+.key-signature-picker-item-stack {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+  align-items: center;
+}
+
+.key-signature-picker-paired-row {
+  display: inline-flex;
+  align-items: baseline;
+  gap: 0;
+  white-space: nowrap;
+}
+
+.key-signature-picker-key {
+  display: inline-flex;
+  align-items: center;
+}
+
+.key-signature-picker-divider {
+  color: #b8b8b8;
+  font-size: 9px;
+  line-height: 1;
+  padding: 0 1px;
+}
+
+.key-signature-picker-button-paired {
+  padding: 0 2px;
+  border-radius: 999px;
+}
+
+.key-signature-picker-button {
+  appearance: none;
+  -webkit-appearance: none;
+  border: none;
+  background: transparent;
+  background-color: transparent;
+  box-shadow: none;
+  padding: 0;
+  font: inherit;
+  cursor: pointer;
+  line-height: 1;
+  transition: transform 0.16s ease, opacity 0.16s ease, text-shadow 0.16s ease;
+}
+
+.key-signature-picker-button:hover {
+  background: transparent;
+  background-color: transparent;
+  transform: scale(1.05);
+  text-shadow: 0 0 10px currentColor;
+}
+
+.key-signature-picker-button:focus-visible {
+  outline: 2px solid #2d2d2d;
+  outline-offset: 4px;
+  border-radius: 6px;
+}
+
+.key-signature-picker-button:focus,
+.key-signature-picker-button:active {
+  background: transparent;
+  background-color: transparent;
+  box-shadow: none;
+}
+
+.key-signature-picker-button.major {
+  color: #e08b94;
+  font-size: 13px;
+}
+
+.key-signature-picker-button.minor {
+  color: #8fba88;
+  font-size: 11px;
+}
+
+.key-signature-picker-button.selected {
+  text-decoration: underline;
+  text-underline-offset: 3px;
+}
+
+.key-signature-picker-label {
+  display: inline-flex;
+  align-items: flex-start;
+}
+
+.key-signature-picker-accidental {
+  font-size: 0.58em;
+  line-height: 1;
+  margin-left: 1px;
+  position: relative;
+  top: -0.15em;
+}
+
+@media (max-width: 900px) {
+  .key-signature-popup {
+    min-width: min(92vw, 270px);
+    padding: 12px 10px 10px;
+  }
+
+  .key-signature-picker {
+    width: min(82vw, 252px);
+    height: min(72vw, 224px);
+  }
+
+  .key-signature-picker-button.major {
+    font-size: 12px;
+  }
+
+  .key-signature-picker-button.minor {
+    font-size: 10px;
+  }
+}
diff --git a/src/components/KeySignaturePickerPopup.test.tsx b/src/components/KeySignaturePickerPopup.test.tsx
new file mode 100644
index 0000000..cb099c6
--- /dev/null
+++ b/src/components/KeySignaturePickerPopup.test.tsx
@@ -0,0 +1,80 @@
+import React from 'react';
+import { fireEvent, render, screen } from '@testing-library/react';
+import { describe, expect, it, vi } from 'vitest';
+import KeySignaturePickerPopup from './KeySignaturePickerPopup';
+
+vi.mock('vexflow', () => {
+  class MockRenderer {
+    static Backends = { SVG: 'svg' };
+    private readonly host: HTMLElement;
+
+    constructor(host: HTMLElement) {
+      this.host = host;
+    }
+
+    resize() {}
+
+    getContext() {
+      const svg = document.createElementNS('http://www.w3.org/2000/svg', 'svg');
+      this.host.appendChild(svg);
+      return {};
+    }
+  }
+
+  class MockStave {
+    constructor() {}
+
+    addClef() {
+      return this;
+    }
+
+    addKeySignature() {
+      return this;
+    }
+
+    setContext() {
+      return this;
+    }
+
+    draw() {
+      return this;
+    }
+  }
+
+  return {
+    Renderer: MockRenderer,
+    Stave: MockStave,
+  };
+});
+
+describe('KeySignaturePickerPopup', () => {
+  it('selects major and minor key signatures', () => {
+    const onChange = vi.fn();
+    render(<KeySignaturePickerPopup value="C major" onChange={onChange} />);
+
+    fireEvent.click(screen.getByRole('button', { name: 'Select G major' }));
+    fireEvent.click(screen.getByRole('button', { name: 'Select E minor' }));
+
+    expect(onChange).toHaveBeenNthCalledWith(1, 'G major');
+    expect(onChange).toHaveBeenNthCalledWith(2, 'E minor');
+  });
+
+  it('renders both exact options for shared enharmonic slots', () => {
+    render(<KeySignaturePickerPopup value="C major" onChange={vi.fn()} />);
+
+    expect(screen.getByRole('button', { name: 'Select Gb major or F# major' })).toBeInTheDocument();
+    expect(screen.getByRole('button', { name: 'Select Eb minor or D# minor' })).toBeInTheDocument();
+  });
+
+  it('toggles between paired enharmonic options from a single button', () => {
+    const onChange = vi.fn();
+    const { rerender } = render(<KeySignaturePickerPopup value="C major" onChange={onChange} />);
+
+    fireEvent.click(screen.getByRole('button', { name: 'Select Gb major or F# major' }));
+    expect(onChange).toHaveBeenCalledWith('Gb major');
+
+    rerender(<KeySignaturePickerPopup value="Gb major" onChange={onChange} />);
+    fireEvent.click(screen.getByRole('button', { name: 'Select Gb major or F# major' }));
+    expect(onChange).toHaveBeenLastCalledWith('F# major');
+  });
+});
diff --git a/src/components/KeySignaturePickerPopup.tsx b/src/components/KeySignaturePickerPopup.tsx
new file mode 100644
index 0000000..38d22a0
--- /dev/null
+++ b/src/components/KeySignaturePickerPopup.tsx
@@ -0,0 +1,164 @@
+import React from 'react';
+import type { KeySignature } from '../core/KGProject';
+import { buildKeySignatureCircleSlots } from './keySignaturePickerLayout';
+import './KeySignaturePickerPopup.css';
+
+interface KeySignaturePickerPopupProps {
+  value: KeySignature;
+  // Keep the callback typed to project key signatures without duplicating the union locally.
+  // eslint-disable-next-line no-unused-vars
+  onChange: (keySignature: KeySignature) => void;
+}
+
+const OUTER_RADIUS = 87;
+const INNER_RADIUS = 55;
+const COUNT_RADIUS = 69;
+const CENTER_X = 126;
+const CENTER_Y = 112;
+
+function getPosition(angleDeg: number, radius: number) {
+  const angle = (angleDeg * Math.PI) / 180;
+  return {
+    left: CENTER_X + radius * Math.cos(angle),
+    top: CENTER_Y + radius * Math.sin(angle),
+  };
+}
+
+function splitAccidental(label: string): { base: string; accidental: string | null } {
+  if (label.includes('#')) {
+    return { base: label.replace('#', ''), accidental: '♯' };
+  }
+  if (label.includes('b')) {
+    return { base: label.replace('b', ''), accidental: '♭' };
+  }
+  return { base: label, accidental: null };
+}
+
+const KeyLabel: React.FC<{ label: string }> = ({ label }) => {
+  const { base, accidental } = splitAccidental(label);
+  return (
+    <span className="key-signature-picker-label">
+      <span>{base}</span>
+      {accidental && <sup className="key-signature-picker-accidental">{accidental}</sup>}
+    </span>
+  );
+};
+
+const CombinedKeyLabel: React.FC<{ labels: string[] }> = ({ labels }) => (
+  <span className="key-signature-picker-label">
+    {labels.map((label, index) => (
+      <React.Fragment key={label}>
+        <KeyLabel label={label} />
+        {index < labels.length - 1 && <span className="key-signature-picker-divider">/</span>}
+      </React.Fragment>
+    ))}
+  </span>
+);
+
+const KeySignaturePickerPopup: React.FC<KeySignaturePickerPopupProps> = ({ value, onChange }) => {
+  const slots = React.useMemo(() => buildKeySignatureCircleSlots(), []);
+
+  const handlePairedSelection = React.useCallback((keySignatures: KeySignature[]) => {
+    if (keySignatures.length === 0) {
+      return;
+    }
+
+    const activeIndex = keySignatures.indexOf(value);
+    if (activeIndex >= 0) {
+      onChange(keySignatures[(activeIndex + 1) % keySignatures.length]);
+      return;
+    }
+
+    onChange(keySignatures[0]);
+  }, [onChange, value]);
+
+  return (
+    <div className="key-signature-popup">
+      <div className="key-signature-picker">
+        <div className="key-signature-picker-ring" aria-hidden="true" />
+        <h3 className="key-signature-picker-title major">Major</h3>
+        <h3 className="key-signature-picker-title minor">Minor</h3>
+        {slots.map((slot) => {
+          const outerPosition = getPosition(slot.angleDeg, OUTER_RADIUS);
+          const innerPosition = getPosition(slot.angleDeg, INNER_RADIUS);
+          const countPosition = getPosition(slot.angleDeg, COUNT_RADIUS);
+
+          return (
+            <React.Fragment key={slot.id}>
+              <div
+                className="key-signature-picker-slot key-signature-picker-slot-outer"
+                style={outerPosition}
+              >
+                {slot.outerItems.length > 1 ? (
+                  <button
+                    type="button"
+                    className={`key-signature-picker-button key-signature-picker-button-paired major ${slot.outerItems.some(item => item.keySignature === value) ? 'selected' : ''}`.trim()}
+                    onClick={() => handlePairedSelection(slot.outerItems.map(item => item.keySignature))}
+                    aria-label={`Select ${slot.outerItems.map(item => item.keySignature).join(' or ')}`}
+                    title={slot.outerItems.map(item => item.keySignature).join(' / ')}
+                  >
+                    <CombinedKeyLabel labels={slot.outerItems.map(item => item.label)} />
+                  </button>
+                ) : (
+                  <div className="key-signature-picker-item-stack">
+                    {slot.outerItems.map((item) => (
+                      <div className="key-signature-picker-key" key={item.keySignature}>
+                        <button
+                          type="button"
+                          className={`key-signature-picker-button major ${value === item.keySignature ? 'selected' : ''}`.trim()}
+                          onClick={() => onChange(item.keySignature)}
+                          aria-label={`Select ${item.keySignature}`}
+                        >
+                          <KeyLabel label={item.label} />
+                        </button>
+                      </div>
+                    ))}
+                  </div>
+                )}
+              </div>
+              <div
+                className="key-signature-picker-slot key-signature-picker-slot-inner"
+                style={innerPosition}
+              >
+                {slot.innerItems.length > 1 ? (
+                  <button
+                    type="button"
+                    className={`key-signature-picker-button key-signature-picker-button-paired minor ${slot.innerItems.some(item => item.keySignature === value) ? 'selected' : ''}`.trim()}
+                    onClick={() => handlePairedSelection(slot.innerItems.map(item => item.keySignature))}
+                    aria-label={`Select ${slot.innerItems.map(item => item.keySignature).join(' or ')}`}
+                    title={slot.innerItems.map(item => item.keySignature).join(' / ')}
+                  >
+                    <CombinedKeyLabel labels={slot.innerItems.map(item => item.label)} />
+                  </button>
+                ) : (
+                  <div className="key-signature-picker-item-stack">
+                    {slot.innerItems.map((item) => (
+                      <button
+                        type="button"
+                        key={item.keySignature}
+                        className={`key-signature-picker-button minor ${value === item.keySignature ? 'selected' : ''}`.trim()}
+                        onClick={() => onChange(item.keySignature)}
+                        aria-label={`Select ${item.keySignature}`}
+                      >
+                        <KeyLabel label={item.label} />
+                      </button>
+                    ))}
+                  </div>
+                )}
+              </div>
+              <div
+                className="key-signature-picker-slot key-signature-picker-slot-count"
+                style={countPosition}
+                aria-hidden="true"
+              >
+                {slot.accidentalLabel}
+              </div>
+            </React.Fragment>
+          );
+        })}
+      </div>
+    </div>
+  );
+};
+
+export default KeySignaturePickerPopup;
diff --git a/src/components/Toolbar.css b/src/components/Toolbar.css
index aac08f2..efed859 100644
--- a/src/components/Toolbar.css
+++ b/src/components/Toolbar.css
@@ -195,29 +195,57 @@
   color: #fff;
 }
 
-.current-key-signature {
+.toolbar button.current-key-signature {
+  appearance: none;
+  border: none;
+  background: transparent;
+  color: inherit;
+  display: inline-block;
+  width: auto;
+  height: auto;
+  min-height: 0;
+  margin: 0;
   font-family: 'Courier New', 'Monaco', 'Menlo', 'Ubuntu Mono', monospace;
   font-weight: normal;
   letter-spacing: 0.5px;
+  line-height: 1;
   cursor: pointer;
   transition: background-color 0.2s ease;
   padding: 2px 4px;
   border-radius: 2px;
+  white-space: nowrap;
 }
 
-.current-key-signature:hover {
+.toolbar button.current-key-signature:hover {
   background-color: #4a4a4a;
   color: #fff;
 }
 
+.toolbar button.current-key-signature:focus-visible {
+  outline: 1px solid #e0e0e0;
+  outline-offset: 1px;
+}
+
 .export-dropdown .quant-dropdown {
   width: 250px;
   left: 0;
 }
 
-.key-signature-dropdown .quant-dropdown {
-  width: 100px;
-  left: 0;
+.key-signature-popup-anchor {
+  display: inline-flex;
+}
+
+.key-signature-popup-surface {
+  --floating-popup-bg: #2d2d2d;
+  --floating-popup-border: #555;
+}
+
+.key-signature-popup-panel {
+  overflow: visible;
+}
+
+.key-signature-popup-arrow {
+  left: 50%;
 }
 
 /* Clickable zoom styling */
diff --git a/src/components/Toolbar.test.tsx b/src/components/Toolbar.test.tsx
index 871d847..a254d3e 100644
--- a/src/components/Toolbar.test.tsx
+++ b/src/components/Toolbar.test.tsx
@@ -62,6 +62,7 @@ const storeState = {
 };
 
 type StoreState = typeof storeState;
+// eslint-disable-next-line no-unused-vars
 type StoreSelector = (state: StoreState) => unknown;
 
 vi.mock('../stores/projectStore', () => ({
@@ -83,7 +84,16 @@ vi.mock('../constants/coreConstants', async (importOriginal) => {
   const actual = await importOriginal<typeof import('../constants/coreConstants')>();
   return {
     ...actual,
-    KEY_SIGNATURE_MAP: { ...actual.KEY_SIGNATURE_MAP, 'C major': [] },
+    KEY_SIGNATURE_MAP: {
+      ...actual.KEY_SIGNATURE_MAP,
+      'C major': actual.KEY_SIGNATURE_MAP['C major'],
+      'G major': actual.KEY_SIGNATURE_MAP['G major'],
+      'E minor': actual.KEY_SIGNATURE_MAP['E minor'],
+      'Gb major': actual.KEY_SIGNATURE_MAP['Gb major'],
+      'F# major': actual.KEY_SIGNATURE_MAP['F# major'],
+      'Eb minor': actual.KEY_SIGNATURE_MAP['Eb minor'],
+      'D# minor': actual.KEY_SIGNATURE_MAP['D# minor'],
+    },
   };
 });
 
@@ -125,7 +135,6 @@ vi.mock('../util/midiUtil', () => ({
   convertMidiToProject: vi.fn(),
 }));
 vi.mock('../core/audio-interface/KGOfflineRenderer', () => ({ KGOfflineRenderer: { instance: vi.fn(() => ({})) } }));
-vi.mock('./common/KGDropdown', () => ({ default: () => null }));
 vi.mock('./common/FileImportModal', () => ({ default: () => null }));
 vi.mock('./common/LoadingOverlay', () => ({ default: () => null }));
 vi.mock('./common/OpenProjectModal', () => ({ default: () => null }));
@@ -140,6 +149,49 @@ vi.mock('../util/dialogUtil', () => ({
   showPrompt: vi.fn(),
   showTimeSigPrompt: vi.fn(),
 }));
+vi.mock('vexflow', () => {
+  class MockRenderer {
+    static Backends = { SVG: 'svg' };
+    private readonly host: HTMLElement;
+
+    constructor(host: HTMLElement) {
+      this.host = host;
+    }
+
+    resize() {}
+
+    getContext() {
+      const svg = document.createElementNS('http://www.w3.org/2000/svg', 'svg');
+      this.host.appendChild(svg);
+      return {};
+    }
+  }
+
+  class MockStave {
+    constructor() {}
+
+    addClef() {
+      return this;
+    }
+
+    addKeySignature() {
+      return this;
+    }
+
+    setContext() {
+      return this;
+    }
+
+    draw() {
+      return this;
+    }
+  }
+
+  return {
+    Renderer: MockRenderer,
+    Stave: MockStave,
+  };
+});
 
 describe('Toolbar settings side-panel behavior', () => {
   beforeEach(() => {
@@ -154,6 +206,8 @@ describe('Toolbar settings side-panel behavior', () => {
     storeState.showChatBox = true;
     storeState.showKGOnePanel = true;
     storeState.showEventListPanel = false;
+    storeState.keySignature = 'C major';
+    storeState.setKeySignature.mockClear();
   });
 
   it('suppresses active styling for side-panel buttons while Settings is visible', () => {
@@ -193,4 +247,30 @@ describe('Toolbar settings side-panel behavior', () => {
       expect(storeState.setStatus).toHaveBeenCalledWith('Split 1 note at beat 12.00');
     });
   });
+
+  it('opens and closes the key signature popup from the toolbar trigger', () => {
+    render(<Toolbar />);
+
+    fireEvent.click(screen.getByRole('button', { name: /choose key signature/i }));
+    expect(screen.getByRole('dialog')).toBeInTheDocument();
+    expect(screen.getByText('Major')).toBeInTheDocument();
+
+    fireEvent.keyDown(document, { key: 'Escape' });
+    expect(screen.queryByRole('dialog')).not.toBeInTheDocument();
+  });
+
+  it('updates the store when a major or minor key is selected', () => {
+    render(<Toolbar />);
+
+    fireEvent.click(screen.getByRole('button', { name: /choose key signature/i }));
+    fireEvent.click(screen.getByRole('button', { name: 'Select G major' }));
+    expect(storeState.setKeySignature).toHaveBeenCalledWith('G major');
+    expect(storeState.setStatus).toHaveBeenCalledWith('Key signature changed to G major');
+    expect(screen.queryByRole('dialog')).not.toBeInTheDocument();
+
+    fireEvent.click(screen.getByRole('button', { name: /choose key signature/i }));
+    fireEvent.click(screen.getByRole('button', { name: 'Select E minor' }));
+    expect(storeState.setKeySignature).toHaveBeenCalledWith('E minor');
+    expect(storeState.setStatus).toHaveBeenCalledWith('Key signature changed to E minor');
+  });
 });
diff --git a/src/components/Toolbar.tsx b/src/components/Toolbar.tsx
index 19f4c66..00bd63c 100644
--- a/src/components/Toolbar.tsx
+++ b/src/components/Toolbar.tsx
@@ -24,12 +24,13 @@ import { KGMainContentState } from '../core/state/KGMainContentState';
 import { regionDeleteManager } from '../util/regionDeleteUtil';
 import { handleCopyOperation, handlePasteOperation } from '../util/copyPasteUtil';
 import { convertProjectToMidi, convertMidiToProject } from '../util/midiUtil';
-import { KEY_SIGNATURE_MAP } from '../constants/coreConstants';
 import { KGOfflineRenderer } from '../core/audio-interface/KGOfflineRenderer';
 import KGDropdown from './common/KGDropdown';
+import FloatingPopup from './common/FloatingPopup';
 import FileImportModal from './common/FileImportModal';
 import LoadingOverlay from './common/LoadingOverlay';
 import OpenProjectModal from './common/OpenProjectModal';
+import KeySignaturePickerPopup from './KeySignaturePickerPopup';
 import { clearChatHistoryAndUI } from '../util/chatUtil';
 import PianoIcon from './common/icons/PianoIcon';
 import MetronomeIcon from './common/icons/MetronomeIcon';
@@ -91,9 +92,6 @@ const Toolbar: React.FC = () => {
     return () => document.removeEventListener('mousedown', handleClickOutside);
   }, [showZoomSlider]);
 
-  // Key signature options
-  const keySignatureOptions = Object.keys(KEY_SIGNATURE_MAP) as KeySignature[];
-
   // Export options
   const exportOptions = ["Export to KGStudio file", "Export to MIDI file", "Export to WAV", "Export to MP3"];
   const lastSelectedRegionId = selectedRegionIds[selectedRegionIds.length - 1] ?? null;
@@ -280,7 +278,7 @@ const Toolbar: React.FC = () => {
     }
   };
 
-  const handleConfirmOpenProject = async (_projectNameToLoad: string) => {
+  const handleConfirmOpenProject = async () => {
     const confirmed = await showConfirm(
       'Open this project? Any unsaved changes in the current project will be lost.'
     );
@@ -1154,25 +1152,29 @@ const Toolbar: React.FC = () => {
               <span className='current-time-signature' onClick={handleTimeSignatureClick} style={{ cursor: 'pointer' }}>{timeSignature.numerator + "/" + timeSignature.denominator}</span>
             </div>
             <div className="transport-item" style={{ position: 'relative' }}>
-              <span
-                className='current-key-signature'
-                onClick={() => setShowKeySignatureDropdown(!showKeySignatureDropdown)}
-                style={{ cursor: 'pointer' }}
+              <FloatingPopup
+                isOpen={showKeySignatureDropdown}
+                onClose={() => setShowKeySignatureDropdown(false)}
+                placement="bottom"
+                className="key-signature-popup-anchor"
+                contentClassName="key-signature-popup-surface"
+                panelClassName="key-signature-popup-panel"
+                arrowClassName="key-signature-popup-arrow"
+                trigger={(
+                  <button
+                    type="button"
+                    className='current-key-signature'
+                    onClick={() => setShowKeySignatureDropdown((current) => !current)}
+                    aria-haspopup="dialog"
+                    aria-expanded={showKeySignatureDropdown}
+                    aria-label={`Choose key signature, current ${keySignature}`}
+                  >
+                    {keySignature}
+                  </button>
+                )}
               >
-                {keySignature}
-              </span>
-              <div style={{ position: 'absolute', top: '100%', left: 0, zIndex: 10000 }}>
-                <KGDropdown
-                  options={keySignatureOptions}
-                  value={keySignature}
-                  onChange={handleKeySignatureChange}
-                  label="Key Signature"
-                  hideButton={true}
-                  isOpen={showKeySignatureDropdown}
-                  onToggle={setShowKeySignatureDropdown}
-                  className="key-signature-dropdown"
-                />
-              </div>
+                <KeySignaturePickerPopup value={keySignature} onChange={handleKeySignatureChange} />
+              </FloatingPopup>
             </div>
           </div>
           <button
diff --git a/src/components/common/FloatingPopup.css b/src/components/common/FloatingPopup.css
new file mode 100644
index 0000000..a5ce88b
--- /dev/null
+++ b/src/components/common/FloatingPopup.css
@@ -0,0 +1,57 @@
+.floating-popup {
+  position: relative;
+  display: inline-flex;
+  align-items: center;
+}
+
+.floating-popup-trigger {
+  display: inline-flex;
+  align-items: center;
+}
+
+.floating-popup-surface {
+  --floating-popup-bg: #2d2d2d;
+  --floating-popup-border: #4a4a4a;
+  --floating-popup-shadow: 0 18px 36px rgba(0, 0, 0, 0.28);
+  position: absolute;
+  z-index: 10000;
+}
+
+.floating-popup-surface[data-placement='bottom'] {
+  top: calc(100% + 14px);
+  left: 50%;
+  transform: translateX(-50%);
+}
+
+.floating-popup-panel {
+  position: relative;
+  background: var(--floating-popup-bg);
+  border: 1px solid var(--floating-popup-border);
+  border-radius: 28px;
+  box-shadow: var(--floating-popup-shadow);
+}
+
+.floating-popup-panel::before {
+  content: '';
+  position: absolute;
+  top: -1px;
+  left: calc(50% - 12px);
+  width: 24px;
+  height: 1px;
+  background: var(--floating-popup-bg);
+}
+
+.floating-popup-arrow {
+  position: absolute;
+  width: 20px;
+  height: 20px;
+  background: var(--floating-popup-bg);
+  transform: translateX(-50%) rotate(45deg);
+}
+
+.floating-popup-arrow[data-placement='bottom'] {
+  top: -9px;
+  left: 50%;
+  border-top: 1px solid var(--floating-popup-border);
+  border-left: 1px solid var(--floating-popup-border);
+}
\ No newline at end of file
diff --git a/src/components/common/FloatingPopup.test.tsx b/src/components/common/FloatingPopup.test.tsx
new file mode 100644
index 0000000..cb2a8fc
--- /dev/null
+++ b/src/components/common/FloatingPopup.test.tsx
@@ -0,0 +1,29 @@
+import React from 'react';
+import { fireEvent, render, screen } from '@testing-library/react';
+import { describe, expect, it, vi } from 'vitest';
+import FloatingPopup from './FloatingPopup';
+
+describe('FloatingPopup', () => {
+  it('closes on outside click and escape', () => {
+    const onClose = vi.fn();
+    render(
+      <div>
+        <FloatingPopup
+          isOpen={true}
+          onClose={onClose}
+          trigger={<button type="button">Toggle</button>}
+        >
+          <div>Popup body</div>
+        </FloatingPopup>
+        <button type="button">Outside</button>
+      </div>
+    );
+
+    expect(screen.getByRole('dialog')).toBeInTheDocument();
+
+    fireEvent.mouseDown(screen.getByRole('button', { name: 'Outside' }));
+    fireEvent.keyDown(document, { key: 'Escape' });
+
+    expect(onClose).toHaveBeenCalledTimes(2);
+  });
+});
diff --git a/src/components/common/FloatingPopup.tsx b/src/components/common/FloatingPopup.tsx
new file mode 100644
index 0000000..c746d8b
--- /dev/null
+++ b/src/components/common/FloatingPopup.tsx
@@ -0,0 +1,85 @@
+import React from 'react';
+import './FloatingPopup.css';
+
+type FloatingPopupPlacement = 'bottom' | 'top' | 'left' | 'right';
+
+interface FloatingPopupProps {
+  isOpen: boolean;
+  onClose: () => void;
+  trigger: React.ReactNode;
+  children: React.ReactNode;
+  placement?: FloatingPopupPlacement;
+  className?: string;
+  triggerClassName?: string;
+  contentClassName?: string;
+  panelClassName?: string;
+  arrowClassName?: string;
+}
+
+const FloatingPopup: React.FC<FloatingPopupProps> = ({
+  isOpen,
+  onClose,
+  trigger,
+  children,
+  placement = 'bottom',
+  className = '',
+  triggerClassName = '',
+  contentClassName = '',
+  panelClassName = '',
+  arrowClassName = '',
+}) => {
+  const rootRef = React.useRef<HTMLDivElement | null>(null);
+
+  React.useEffect(() => {
+    if (!isOpen) {
+      return;
+    }
+
+    const handleMouseDown = (event: MouseEvent) => {
+      if (rootRef.current && !rootRef.current.contains(event.target as Node)) {
+        onClose();
+      }
+    };
+
+    const handleKeyDown = (event: KeyboardEvent) => {
+      if (event.key === 'Escape') {
+        onClose();
+      }
+    };
+
+    document.addEventListener('mousedown', handleMouseDown);
+    document.addEventListener('keydown', handleKeyDown);
+
+    return () => {
+      document.removeEventListener('mousedown', handleMouseDown);
+      document.removeEventListener('keydown', handleKeyDown);
+    };
+  }, [isOpen, onClose]);
+
+  return (
+    <div className={`floating-popup ${className}`.trim()} ref={rootRef}>
+      <div className={`floating-popup-trigger ${triggerClassName}`.trim()}>
+        {trigger}
+      </div>
+      {isOpen && (
+        <div
+          className={`floating-popup-surface ${contentClassName}`.trim()}
+          data-placement={placement}
+          role="dialog"
+          aria-modal="false"
+        >
+          <div
+            className={`floating-popup-arrow ${arrowClassName}`.trim()}
+            data-placement={placement}
+            aria-hidden="true"
+          />
+          <div className={`floating-popup-panel ${panelClassName}`.trim()}>
+            {children}
+          </div>
+        </div>
+      )}
+    </div>
+  );
+};
+
+export default FloatingPopup;
diff --git a/src/components/common/index.ts b/src/components/common/index.ts
index 093aaf4..e43862f 100644
--- a/src/components/common/index.ts
+++ b/src/components/common/index.ts
@@ -5,5 +5,6 @@ export { default as LoadingOverlay } from './LoadingOverlay';
 export { default as OpenProjectModal } from './OpenProjectModal';
 export { default as DialogProvider } from './DialogProvider';
 export { default as TrackCreateDialog } from './TrackCreateDialog';
+export { default as FloatingPopup } from './FloatingPopup';
 export { showAlert, showConfirm, showPrompt, showTimeSigPrompt } from '../../util/dialogUtil';
 export type { ConfirmOptions, PromptOptions, TimeSigResult } from '../../util/dialogUtil';
diff --git a/src/components/keySignaturePickerLayout.test.ts b/src/components/keySignaturePickerLayout.test.ts
new file mode 100644
index 0000000..a568587
--- /dev/null
+++ b/src/components/keySignaturePickerLayout.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest';
+import { buildKeySignatureCircleSlots } from './keySignaturePickerLayout';
+
+describe('keySignaturePickerLayout', () => {
+  it('builds shared enharmonic slots with both exact key signatures', () => {
+    const slots = buildKeySignatureCircleSlots();
+    const sharedMajorSlot = slots.find(slot => slot.outerItems.some(item => item.keySignature === 'Gb major'));
+    const sharedMinorSlot = slots.find(slot => slot.innerItems.some(item => item.keySignature === 'D# minor'));
+
+    expect(sharedMajorSlot?.outerItems.map(item => item.keySignature)).toEqual(['Gb major', 'F# major']);
+    expect(sharedMinorSlot?.innerItems.map(item => item.keySignature)).toEqual(['Eb minor', 'D# minor']);
+  });
+
+  it('uses accidental counts derived from the key signature map', () => {
+    const slots = buildKeySignatureCircleSlots();
+    const cSlot = slots.find(slot => slot.id === 'c');
+    const gbSlot = slots.find(slot => slot.id === 'gb-fsharp');
+
+    expect(cSlot?.accidentalLabel).toBe('natural');
+    expect(gbSlot?.accidentalLabel).toBe('6♭ / 6♯');
+  });
+});
diff --git a/src/components/keySignaturePickerLayout.ts b/src/components/keySignaturePickerLayout.ts
new file mode 100644
index 0000000..e1a0354
--- /dev/null
+++ b/src/components/keySignaturePickerLayout.ts
@@ -0,0 +1,92 @@
+import { KEY_SIGNATURE_MAP } from '../constants/coreConstants';
+import type { KeySignature } from '../core/KGProject';
+
+type KeyQuality = 'major' | 'minor';
+
+interface KeySignatureSlotDefinition {
+  id: string;
+  majorKeys: KeySignature[];
+  minorKeys: KeySignature[];
+}
+
+export interface KeySignatureCircleItem {
+  keySignature: KeySignature;
+  label: string;
+  quality: KeyQuality;
+}
+
+export interface KeySignatureCircleSlot {
+  id: string;
+  angleDeg: number;
+  outerItems: KeySignatureCircleItem[];
+  innerItems: KeySignatureCircleItem[];
+  accidentalLabel: string;
+}
+
+const CIRCLE_SLOT_DEFINITIONS: KeySignatureSlotDefinition[] = [
+  { id: 'c', majorKeys: ['C major'], minorKeys: ['A minor'] },
+  { id: 'g', majorKeys: ['G major'], minorKeys: ['E minor'] },
+  { id: 'd', majorKeys: ['D major'], minorKeys: ['B minor'] },
+  { id: 'a', majorKeys: ['A major'], minorKeys: ['F# minor'] },
+  { id: 'e', majorKeys: ['E major'], minorKeys: ['C# minor'] },
+  { id: 'b-cb', majorKeys: ['B major', 'Cb major'], minorKeys: ['G# minor', 'Ab minor'] },
+  { id: 'gb-fsharp', majorKeys: ['Gb major', 'F# major'], minorKeys: ['Eb minor', 'D# minor'] },
+  { id: 'db-csharp', majorKeys: ['Db major', 'C# major'], minorKeys: ['Bb minor', 'A# minor'] },
+  { id: 'ab', majorKeys: ['Ab major'], minorKeys: ['F minor'] },
+  { id: 'eb', majorKeys: ['Eb major'], minorKeys: ['C minor'] },
+  { id: 'bb', majorKeys: ['Bb major'], minorKeys: ['G minor'] },
+  { id: 'f', majorKeys: ['F major'], minorKeys: ['D minor'] },
+];
+
+function assertKnownKeySignature(keySignature: KeySignature): KeySignature {
+  if (!(keySignature in KEY_SIGNATURE_MAP)) {
+    throw new Error(`Unknown key signature in circle layout: ${keySignature}`);
+  }
+
+  return keySignature;
+}
+
+function getLabelForKeySignature(keySignature: KeySignature): string {
+  const [tonic, quality] = keySignature.split(' ');
+  return quality === 'major' ? tonic : tonic.toLowerCase();
+}
+
+function formatAccidentalCount(keySignatures: KeySignature[]): string {
+  const segments = keySignatures.map((keySignature) => {
+    const entry = KEY_SIGNATURE_MAP[keySignature];
+    if (entry.sharps > 0) {
+      return `${entry.sharps}♯`;
+    }
+    if (entry.flats > 0) {
+      return `${entry.flats}♭`;
+    }
+    return 'natural';
+  });
+
+  return [...new Set(segments)].join(' / ');
+}
+
+export function buildKeySignatureCircleSlots(): KeySignatureCircleSlot[] {
+  const totalSlots = CIRCLE_SLOT_DEFINITIONS.length;
+
+  return CIRCLE_SLOT_DEFINITIONS.map((slot, index) => {
+    const outerItems = slot.majorKeys.map((keySignature) => ({
+      keySignature: assertKnownKeySignature(keySignature),
+      label: getLabelForKeySignature(keySignature),
+      quality: 'major' as const,
+    }));
+    const innerItems = slot.minorKeys.map((keySignature) => ({
+      keySignature: assertKnownKeySignature(keySignature),
+      label: getLabelForKeySignature(keySignature),
+      quality: 'minor' as const,
+    }));
+
+    return {
+      id: slot.id,
+      angleDeg: -90 + (360 / totalSlots) * index,
+      outerItems,
+      innerItems,
+      accidentalLabel: formatAccidentalCount(slot.majorKeys),
+    };
+  });
+}