feat: add scale highlighting to Piano Roll grid

src/App.css

@@ -863,19 +863,8 @@ body {
   width: 100%;
   height: 100%;
   position: relative;
-  background-size: var(--region-grid-beat-width) var(--region-piano-key-height), var(--region-grid-bar-width) var(--region-piano-key-height);
-  background-image: 
-    /* Vertical lines for beats */
-    linear-gradient(to right, 
-      transparent calc(var(--region-grid-beat-width) - 1px), #444 calc(var(--region-grid-beat-width) - 1px), #444 var(--region-grid-beat-width), 
-      transparent calc(var(--region-grid-beat-width)), transparent calc(var(--region-grid-beat-width) * 2 - 1px), #444 calc(var(--region-grid-beat-width) * 2 - 1px), #444 calc(var(--region-grid-beat-width) * 2),
-      transparent calc(var(--region-grid-beat-width) * 2), transparent calc(var(--region-grid-beat-width) * 3 - 1px), #444 calc(var(--region-grid-beat-width) * 3 - 1px), #444 calc(var(--region-grid-beat-width) * 3),
-      transparent calc(var(--region-grid-beat-width) * 3), transparent calc(var(--region-grid-beat-width) * 4 - 1px), #3a3a3a calc(var(--region-grid-beat-width) * 4 - 1px), #3a3a3a calc(var(--region-grid-beat-width) * 4)
-    ),
-    /* Horizontal lines for notes */
-    linear-gradient(to bottom,
-      transparent calc(var(--region-piano-key-height) - 1px), #3a3a3a calc(var(--region-piano-key-height) - 1px), #3a3a3a var(--region-piano-key-height)
-    );
+  background-size: var(--region-grid-beat-width) var(--region-piano-key-height), 100% 100%;
+  /* background-image is now set dynamically via React inline styles in PianoGrid component */
 }
 
 .piano-grid.pencil-cursor {

src/App.tsx

@@ -38,13 +38,15 @@ function App() {
       try {
         const response = await fetch(`${import.meta.env.BASE_URL}resources/modes/mode_list.json`);
         const data = await response.json();
-        const modeNames = data.modes.map((mode: { name: string; steps: number[] }) => mode.name);
-        KGPianoRollState.MODE_OPTIONS = modeNames;
-        console.log(`Loaded ${modeNames.length} modes:`, modeNames);
+
+        // Store mode data with id, name, and steps
+        KGPianoRollState.MODE_DATA = data.modes;
+
+        console.log(`Loaded ${data.modes.length} modes:`, data.modes.map((m: { name: string }) => m.name));
       } catch (error) {
         console.error('Failed to load mode list:', error);
         // Fallback to default mode
-        KGPianoRollState.MODE_OPTIONS = ['ionian'];
+        KGPianoRollState.MODE_DATA = [{ id: 'ionian', name: 'Ionian', steps: [2, 2, 1, 2, 2, 2, 1] }];
       }
 
       // Log maxBars to console

src/components/piano-roll/PianoGrid.tsx

@@ -1,8 +1,10 @@
-import React, { useState, useEffect } from 'react';
+import React, { useState, useEffect, useMemo } from 'react';
 import type { MutableRefObject } from 'react';
 import { Playhead } from '../common';
 import SelectionBox from './SelectionBox';
 import { isModifierKeyPressed } from '../../util/osUtil';
+import { generatePianoGridBackground } from '../../util/scaleUtil';
+import type { KeySignature } from '../../core/KGProject';
 
 interface PianoGridProps {
   gridRef: MutableRefObject<HTMLDivElement | null>;
@@ -18,6 +20,8 @@ interface PianoGridProps {
     endY: number;
   };
   regionStartBeat?: number;
+  selectedMode: string;
+  keySignature: KeySignature;
 }
 
 interface CursorPosition {
@@ -35,11 +39,18 @@ const PianoGrid: React.FC<PianoGridProps> = ({
   onMouseDown,
   isBoxSelecting,
   selectionBox,
-  regionStartBeat = 0
+  regionStartBeat = 0,
+  selectedMode,
+  keySignature
 }) => {
   const [cursorPosition, setCursorPosition] = useState<CursorPosition | null>(null);
   const [isModifierPressed, setIsModifierPressed] = useState(false);
 
+  // Generate background with scale highlighting - only regenerate when mode or key changes
+  const backgroundImage = useMemo(() => {
+    return generatePianoGridBackground(selectedMode, keySignature);
+  }, [selectedMode, keySignature]);
+
   // Track modifier key state
   useEffect(() => {
     const handleKeyDown = (e: KeyboardEvent) => {
@@ -119,6 +130,7 @@ const PianoGrid: React.FC<PianoGridProps> = ({
       <div
         className={`piano-grid ${isModifierPressed ? 'pencil-cursor' : ''}`}
         ref={gridRef}
+        style={{ backgroundImage }}
         onDoubleClick={onDoubleClick}
         onClick={onClick}
         onMouseDown={(e) => onMouseDown(e)}

src/components/piano-roll/PianoRoll.tsx

@@ -27,7 +27,7 @@ const PianoRoll: React.FC<PianoRollProps> = ({
   initialPosition,
   initialSize
 }) => {
-  const { maxBars, tracks, updateTrack, timeSignature, showChatBox, showInstrumentSelection } = useProjectStore();
+  const { maxBars, tracks, updateTrack, timeSignature, showChatBox, showInstrumentSelection, keySignature } = useProjectStore();
   
   // Tool state for piano roll
   const [activeTool, setActiveTool] = useState<'pointer' | 'pencil'>('pointer');
@@ -792,6 +792,8 @@ const PianoRoll: React.FC<PianoRollProps> = ({
         tracks={tracks}
         onSetNoteUpdateTrigger={handleSetNoteUpdateTrigger}
         onSetDeleteNotesTrigger={handleSetDeleteNotesTrigger}
+        selectedMode={selectedMode}
+        keySignature={keySignature}
       />
       
       <div 

src/components/piano-roll/PianoRollContent.tsx

@@ -10,6 +10,7 @@ import PianoGridHeader from './PianoGridHeader';
 import PianoGrid from './PianoGrid';
 import { useNoteOperations } from '../../hooks/useNoteOperations';
 import { useNoteSelection } from '../../hooks/useNoteSelection';
+import type { KeySignature } from '../../core/KGProject';
 
 interface PianoRollContentProps {
   contentRef: React.MutableRefObject<HTMLDivElement | null>;
@@ -21,6 +22,8 @@ interface PianoRollContentProps {
   tracks: KGTrack[];
   onSetNoteUpdateTrigger?: (setNoteFn: React.Dispatch<React.SetStateAction<number>>) => void;
   onSetDeleteNotesTrigger?: (deleteFn: () => boolean) => void;
+  selectedMode: string;
+  keySignature: KeySignature;
 }
 
 const PianoRollContent: React.FC<PianoRollContentProps> = ({
@@ -32,7 +35,9 @@ const PianoRollContent: React.FC<PianoRollContentProps> = ({
   updateTrack,
   tracks,
   onSetNoteUpdateTrigger,
-  onSetDeleteNotesTrigger
+  onSetDeleteNotesTrigger,
+  selectedMode,
+  keySignature
 }) => {
   // Get KGCore instance
   const core = KGCore.instance();
@@ -227,6 +232,8 @@ const PianoRollContent: React.FC<PianoRollContentProps> = ({
           isBoxSelecting={isBoxSelectingRef.current}
           selectionBox={selectionBoxRef.current}
           regionStartBeat={activeRegion?.getStartFromBeat() || 0}
+          selectedMode={selectedMode}
+          keySignature={keySignature}
         >
           {memoizedNotes}
         </PianoGrid>

src/components/piano-roll/PianoRollToolbar.tsx

@@ -33,7 +33,7 @@ const PianoRollToolbar: React.FC<PianoRollToolbarProps> = ({
       <div className="toolbar-left">
         {/* Left section with mode dropdown */}
         <KGDropdown
-          options={KGPianoRollState.MODE_OPTIONS}
+          options={KGPianoRollState.MODE_DATA.map(m => ({ label: m.name, value: m.id }))}
           value={selectedMode}
           onChange={(value) => onModeChange(value)}
           label="Mode"

src/core/state/KGPianoRollState.ts

@@ -8,7 +8,7 @@ export class KGPianoRollState {
   public static SNAP_OPTIONS: string[] = ['NO SNAP', '1/3', '1/4', '1/6', '1/8', '1/12', '1/16', '1/24', '1/32'];
   public static QUANT_POS_OPTIONS: string[] = ['1/3', '1/4', '1/6', '1/8', '1/12', '1/16', '1/24', '1/32'];
   public static QUANT_LEN_OPTIONS: string[] = ['1/1', '1/2', '1/3', '1/4', '1/6', '1/8', '1/12', '1/16', '1/24', '1/32'];
-  public static MODE_OPTIONS: string[] = []; // Will be populated from mode_list.json
+  public static MODE_DATA: Array<{ id: string; name: string; steps: number[] }> = []; // Modes with id, display name, and interval steps
 
   private activeTool: string = "pointer";
   private currentSnap: string = "NO SNAP";

src/util/scaleUtil.ts

@@ -0,0 +1,132 @@
+import { KGPianoRollState } from '../core/state/KGPianoRollState';
+import { pianoRollIndexToPitch } from './midiUtil';
+import type { KeySignature } from '../core/KGProject';
+
+/**
+ * Extracts the root note from a key signature string
+ * @param keySignature - Key signature like "C major", "F# minor", "Bb major"
+ * @returns Root note like "C", "F#", "Bb"
+ */
+export const getRootNoteFromKeySignature = (keySignature: KeySignature): string => {
+  // Extract the note before " major" or " minor"
+  const match = keySignature.match(/^([A-G][#b]?)\s+(major|minor)$/);
+  if (!match) {
+    console.warn(`Invalid key signature format: ${keySignature}, defaulting to C`);
+    return 'C';
+  }
+  return match[1];
+};
+
+/**
+ * Converts a note name (without octave) to pitch class (0-11)
+ * @param noteName - Note name like "C", "C#", "Db", "F#"
+ * @returns Pitch class (0=C, 1=C#/Db, 2=D, ..., 11=B)
+ */
+export const noteNameToPitchClass = (noteName: string): number => {
+  const noteMap: { [key: string]: number } = {
+    'C': 0, 'C#': 1, 'Db': 1,
+    'D': 2, 'D#': 3, 'Eb': 3,
+    'E': 4,
+    'F': 5, 'F#': 6, 'Gb': 6,
+    'G': 7, 'G#': 8, 'Ab': 8,
+    'A': 9, 'A#': 10, 'Bb': 10,
+    'B': 11
+  };
+
+  if (!(noteName in noteMap)) {
+    console.warn(`Invalid note name: ${noteName}, defaulting to C (0)`);
+    return 0;
+  }
+
+  return noteMap[noteName];
+};
+
+/**
+ * Calculates the pitch classes (0-11) that belong to a scale
+ * @param rootNote - Root note like "C", "F#", "Bb"
+ * @param modeSteps - Mode interval steps (e.g., [2, 2, 1, 2, 2, 2, 1] for ionian)
+ * @returns Array of pitch classes in the scale
+ */
+export const getScalePitchClasses = (rootNote: string, modeSteps: number[]): number[] => {
+  const rootPitchClass = noteNameToPitchClass(rootNote);
+  const scalePitchClasses: number[] = [rootPitchClass];
+
+  let currentPitch = rootPitchClass;
+  for (const step of modeSteps.slice(0, -1)) { // Exclude last step (returns to root)
+    currentPitch = (currentPitch + step) % 12;
+    scalePitchClasses.push(currentPitch);
+  }
+
+  return scalePitchClasses;
+};
+
+/**
+ * Gets the mode steps for a given mode id
+ * @param modeId - ID of the mode (e.g., "ionian", "aeolian")
+ * @returns Array of interval steps, or default ionian if not found
+ */
+export const getModeSteps = (modeId: string): number[] => {
+  const modeData = KGPianoRollState.MODE_DATA.find(m => m.id === modeId);
+  if (!modeData) {
+    console.warn(`Mode not found: ${modeId}, defaulting to ionian`);
+    return [2, 2, 1, 2, 2, 2, 1]; // Default to ionian (major scale)
+  }
+  return modeData.steps;
+};
+
+/**
+ * Generates the CSS background-image string for the piano grid with scale highlighting
+ * @param selectedMode - Current mode ID (e.g., "ionian", "dorian")
+ * @param keySignature - Current key signature (e.g., "C major", "F# minor")
+ * @returns CSS background-image string with highlighted scale notes
+ */
+export const generatePianoGridBackground = (
+  selectedMode: string,
+  keySignature: KeySignature
+): string => {
+  // Get root note and scale pitch classes
+  const rootNote = getRootNoteFromKeySignature(keySignature);
+  const modeSteps = getModeSteps(selectedMode);
+  const scalePitchClasses = getScalePitchClasses(rootNote, modeSteps);
+
+  // Generate horizontal lines for each of 96 rows (8 octaves)
+  const horizontalLines = Array.from({ length: 96 }, (_, index) => {
+    const pitch = pianoRollIndexToPitch(index);
+    const pitchClass = pitch % 12;
+    const isInScale = scalePitchClasses.includes(pitchClass);
+
+    // Calculate row positions using CSS calc() with --region-piano-key-height variable
+    const rowTop = `calc(var(--region-piano-key-height) * ${index})`;
+    const rowBottomMinusOne = `calc(var(--region-piano-key-height) * ${index + 1} - 1px)`;
+    const rowBottom = `calc(var(--region-piano-key-height) * ${index + 1})`;
+
+    // For scale notes: highlight the full row with a semi-transparent blue background
+    // For non-scale notes: use transparent background with just the separator line
+    if (isInScale) {
+      return `
+        rgba(90, 123, 154, 0.15) ${rowTop},
+        rgba(90, 123, 154, 0.15) ${rowBottomMinusOne},
+        #3a3a3a ${rowBottomMinusOne},
+        #3a3a3a ${rowBottom}
+      `.trim();
+    } else {
+      return `
+        transparent ${rowTop},
+        transparent ${rowBottomMinusOne},
+        #3a3a3a ${rowBottomMinusOne},
+        #3a3a3a ${rowBottom}
+      `.trim();
+    }
+  }).join(',\n');
+
+  // Return complete background-image with vertical and horizontal gradients
+  // Note: Vertical beat lines gradient should be preserved from existing CSS
+  return `
+    linear-gradient(to right,
+      transparent calc(var(--region-grid-beat-width) - 1px),
+      #3a3a3a calc(var(--region-grid-beat-width) - 1px),
+      #3a3a3a var(--region-grid-beat-width)
+    ),
+    linear-gradient(to bottom, ${horizontalLines})
+  `;
+};