feat: added rules row under the main grid bar number row

src/components/MainContent.css

@@ -20,13 +20,14 @@
   top: 50px;
   left: 0;
   width: var(--track-info-panel-width);
-  height: 20px;
+  height: 40px;
   background-color: #2d2d2d;
   border-bottom: 1px solid #3a3a3a;
   border-right: 1px solid #3a3a3a;
   z-index: 1002; /* Higher than other elements to ensure it's always visible */
   display: flex;
   flex-direction: row;
+  align-items: stretch;
 }
 
 /* Offset spacer when instrument selection panel is visible on the left */
@@ -37,7 +38,7 @@
 .bar-numbers {
   position: sticky;
   top: 0;
-  height: 20px;
+  height: 40px;
   display: flex;
   border-bottom: 1px solid #3a3a3a;
   background-color: #2d2d2d;
@@ -56,17 +57,51 @@
   width: var(--track-grid-bar-width);
   flex-shrink: 0;
   flex-grow: 0;
-  height: 20px;
-  text-align: center;
-  font-size: 12px;
+  height: 40px;
   border-right: 1px solid #3a3a3a;
   color: #999;
   display: flex;
+  flex-direction: column;
+  box-sizing: border-box;
+  position: relative;
+}
+
+.bar-number-label {
+  height: 20px;
+  text-align: center;
+  font-size: 12px;
+  display: flex;
   align-items: center;
   justify-content: center;
+}
+
+.bar-beat-markers {
+  position: relative;
+  height: 20px;
+  border-top: 1px solid #3a3a3a;
+  border-bottom: 1px solid #3a3a3a;
+  background-color: #1e1e1e;
   box-sizing: border-box;
 }
 
+.bar-boundary-marker {
+  position: absolute;
+  left: -1px;
+  bottom: 0;
+  width: 1px;
+  height: 100%;
+  background-color: #3a3a3a;
+}
+
+.beat-marker {
+  position: absolute;
+  bottom: 0;
+  width: 1px;
+  height: 50%;
+  background-color: #3a3a3a;
+  transform: translateX(-50%);
+}
+
 .bar-number-cell.looped {
   background-color: #e1ae01;
   color: #1e1e1e;
@@ -91,7 +126,7 @@
    Only shift the fixed spacer when the left panel is present. */
 
 .track-top-spacer {
-  height: 20px;
+  height: 40px;
   border-bottom: 1px solid #3a3a3a;
   background-color: #2d2d2d;
   position: sticky;

src/components/MainContent.test.tsx

@@ -131,6 +131,7 @@ describe('MainContent', () => {
     storeState.selectedRegionIds = [];
     storeState.activeRegionId = null;
     storeState.showPianoRoll = false;
+    storeState.timeSignature = { numerator: 4, denominator: 4 };
     storeState.clearAllSelections.mockClear();
     storeState.setSelectedTrack.mockClear();
     storeState.setShowPianoRoll.mockClear();
@@ -203,4 +204,30 @@ describe('MainContent', () => {
     expect(storeState.setShowPianoRoll).toHaveBeenCalledWith(false);
     expect(storeState.setActiveRegionId).toHaveBeenCalledWith(null);
   });
+
+  it('renders the two-row timeline ruler with add-track buttons and beat markers', () => {
+    const { container } = render(<MainContent />);
+
+    expect(screen.getByRole('button', { name: '+ MIDI' })).toBeInTheDocument();
+    expect(screen.getByRole('button', { name: '+ Audio' })).toBeInTheDocument();
+
+    const barCells = container.querySelectorAll('[data-testid="bar-number-cell"]');
+    expect(barCells).toHaveLength(storeState.maxBars);
+    expect(barCells[0]).toHaveTextContent('1');
+    expect(barCells[storeState.maxBars - 1]).toHaveTextContent(String(storeState.maxBars));
+
+    const beatRows = container.querySelectorAll('[data-testid="bar-beat-markers"]');
+    expect(beatRows).toHaveLength(storeState.maxBars);
+    expect(beatRows[0].querySelectorAll('.beat-marker')).toHaveLength(storeState.timeSignature.numerator - 1);
+    expect(beatRows[0].querySelector('.bar-boundary-marker')).not.toBeNull();
+  });
+
+  it('updates lower-row beat ticks when the time signature numerator changes', () => {
+    storeState.timeSignature = { numerator: 3, denominator: 4 };
+
+    const { container } = render(<MainContent />);
+    const firstBeatRow = container.querySelector('[data-testid="bar-beat-markers"]');
+
+    expect(firstBeatRow?.querySelectorAll('.beat-marker')).toHaveLength(2);
+  });
 });

src/components/MainContent.tsx

@@ -992,6 +992,7 @@ const MainContent: React.FC<MainContentProps> = ({
   }, [calculateBarIndexFromMouse, calculatePlayheadFromMouse, setPlayheadPosition, requestPianoRollScroll]);
 
   const { showInstrumentSelection, isLooping, loopingRange } = useProjectStore();
+  const beatTicksPerBar = Math.max(0, timeSignature.numerator - 1);
 
   // Helper function to check if a bar (0-indexed) is in the loop range
   const isBarInLoopRange = (barIndex: number): boolean => {
@@ -1024,8 +1025,19 @@ const MainContent: React.FC<MainContentProps> = ({
             <div
               key={i}
               className={`bar-number-cell${isBarInLoopRange(i) ? ' looped' : ''}`}
+              data-testid="bar-number-cell"
             >
-              {i + 1}
+              <div className="bar-number-label">{i + 1}</div>
+              <div className="bar-beat-markers" data-testid="bar-beat-markers">
+                <div className="bar-boundary-marker" />
+                {Array.from({ length: beatTicksPerBar }, (_, beatIndex) => (
+                  <div
+                    key={beatIndex}
+                    className="beat-marker"
+                    style={{ left: `${((beatIndex + 1) / timeSignature.numerator) * 100}%` }}
+                  />
+                ))}
+              </div>
             </div>
           ))}
         </div>