fix: DH-21258: Fix column group separators (#2597)

Update `GridUtils.getColumnSeparatorIndex` to take column group depth
into account.

BREAKING CHANGE: `getColumnSeparatorIndex` now requires a `model`
argument.

Author: vbabich

packages/grid/src/GridUtils.test.ts

@@ -1,7 +1,10 @@
+import { TestUtils } from '@deephaven/test-utils';
 import { type AxisRange, type BoundedAxisRange } from './GridAxisRange';
 import { type ModelIndex, type MoveOperation } from './GridMetrics';
 import type GridMetrics from './GridMetrics';
+import type GridModel from './GridModel';
 import GridRange, { type GridRangeIndex } from './GridRange';
+import GridTheme from './GridTheme';
 import GridUtils, { type Token, type TokenBox } from './GridUtils';
 
 function expectModelIndexes(
@@ -1067,3 +1070,145 @@ describe('translateTokenBox', () => {
     expect(GridUtils.translateTokenBox(input, metrics)).toEqual(expectedValue);
   });
 });
+
+describe('getColumnSeparatorIndex', () => {
+  const mockTheme = {
+    ...GridTheme,
+    allowColumnResize: true,
+    headerSeparatorHandleSize: 10,
+    columnHeaderHeight: 30,
+  };
+
+  const createMockMetrics = (
+    columnHeaderMaxDepth = 1
+  ): Partial<GridMetrics> => ({
+    rowHeaderWidth: 50,
+    columnHeaderHeight: 30,
+    columnHeaderMaxDepth,
+    floatingColumns: [],
+    floatingLeftWidth: 0,
+    visibleColumns: [0, 1, 2, 3],
+    allColumnXs: new Map([
+      [0, 0],
+      [1, 100],
+      [2, 200],
+      [3, 300],
+    ]),
+    allColumnWidths: new Map([
+      [0, 100],
+      [1, 100],
+      [2, 100],
+      [3, 100],
+    ]),
+    modelColumns: new Map([
+      [0, 0],
+      [1, 1],
+      [2, 2],
+      [3, 3],
+    ]),
+    // Additional properties needed for getRowAtY (called by getColumnHeaderDepthAtY)
+    gridY: 30,
+    floatingTopRowCount: 0,
+    floatingBottomRowCount: 0,
+    rowCount: 100,
+    visibleRows: [],
+    allRowYs: new Map(),
+    allRowHeights: new Map(),
+  });
+
+  /**
+   * Creates a mock GridModel with grouped column headers for testing
+   */
+  const createMockGroupedGridModel = (
+    headerGroups: Map<number, Map<number, string>>
+  ): GridModel =>
+    TestUtils.createMockProxy<GridModel>({
+      columnCount: 4,
+      rowCount: 100,
+      columnHeaderMaxDepth: headerGroups.size,
+      textForColumnHeader: (column: ModelIndex, depth = 0) =>
+        headerGroups.get(depth)?.get(column) ?? '',
+    });
+
+  const singleLevelHeaderGroups = new Map([
+    [
+      0,
+      new Map([
+        [0, 'A'],
+        [1, 'B'],
+        [2, 'C'],
+        [3, 'D'],
+      ]),
+    ],
+  ]);
+
+  const multiLevelHeaderGroups = new Map([
+    [
+      0,
+      new Map([
+        [0, 'A'],
+        [1, 'B'],
+        [2, 'C'],
+        [3, 'D'],
+      ]),
+    ],
+    [
+      1,
+      new Map([
+        [0, 'Group1'],
+        [1, 'Group1'],
+        [2, 'Group2'],
+        [3, 'Group2'],
+      ]),
+    ],
+  ]);
+
+  it.each([
+    {
+      description: 'detects separator at column boundary',
+      x: 150, // At boundary between column 0 and 1 (100 + 50)
+      y: 15, // Middle of the top header (maxDepth - 1)
+      headerGroups: singleLevelHeaderGroups,
+      maxDepth: 1,
+      expected: 0,
+    },
+    {
+      description: 'detects there is no separator within the column',
+      x: 120, // Within column 1
+      y: 15,
+      headerGroups: singleLevelHeaderGroups,
+      maxDepth: 1,
+      expected: null,
+    },
+    {
+      description:
+        'should return null at depth 1 when no separator exists (columns in same group)',
+      x: 150, // Between column 0 and 1
+      y: 15, // Middle of the top header (maxDepth - 1)
+      headerGroups: multiLevelHeaderGroups,
+      maxDepth: 2,
+      expected: null,
+    },
+    {
+      description: 'should detect separator at depth 1 when groups differ',
+      x: 250, // Between Group1 and Group2
+      y: 15,
+      headerGroups: multiLevelHeaderGroups,
+      maxDepth: 2,
+      expected: 1,
+    },
+  ])('$description', ({ x, y, headerGroups, maxDepth, expected }) => {
+    const metrics = createMockMetrics(maxDepth) as GridMetrics;
+    const model = createMockGroupedGridModel(headerGroups);
+
+    const result = GridUtils.getColumnSeparatorIndex(
+      x,
+      y,
+      metrics,
+      mockTheme,
+      model
+    );
+
+    expect(result).toBe(expected);
+  });
+});

packages/grid/src/GridUtils.ts

@@ -25,6 +25,7 @@ import {
 } from './GridAxisRange';
 import { isExpandableGridModel } from './ExpandableGridModel';
 import { type GridRenderState } from './GridRendererTypes';
+import type GridModel from './GridModel';
 
 export type GridPoint = {
   x: Coordinate;
@@ -453,19 +454,53 @@ export class GridUtils {
     );
   }
 
+  /**
+   * Check if a separator exists between a column and the next column at a given depth.
+   * A separator exists if adjacent columns have different header text at the specified depth.
+   *
+   * @param model The grid model
+   * @param depth The header depth to check at
+   * @param columnIndex The current model column index
+   * @param nextColumnIndex The next model column index (undefined for last column)
+   * @returns true if a separator should be shown, false otherwise
+   */
+  static hasColumnSeparatorAtDepth(
+    model: GridModel,
+    depth: number | undefined,
+    columnIndex: ModelIndex | undefined,
+    nextColumnIndex: ModelIndex | undefined
+  ): boolean {
+    if (depth == null || columnIndex == null) {
+      return false;
+    }
+
+    // Always show separator for the last column
+    if (nextColumnIndex == null) {
+      return true;
+    }
+
+    // A separator exists if adjacent columns have different header text at this depth
+    return (
+      model.textForColumnHeader(columnIndex, depth) !==
+      model.textForColumnHeader(nextColumnIndex, depth)
+    );
+  }
+
   /**
    * Gets the column index if the x/y coordinates provided are close enough to the separator, otherwise null
    * @param x Mouse x coordinate
    * @param y Mouse y coordinate
    * @param metrics The grid metrics
    * @param theme The grid theme with potential user overrides
+   * @param model The grid model
    * @returns Index of the column separator at the coordinates provided, or null if none match
    */
   static getColumnSeparatorIndex(
     x: Coordinate,
     y: Coordinate,
     metrics: GridMetrics,
-    theme: GridTheme
+    theme: GridTheme,
+    model: GridModel
   ): VisibleIndex | null {
     const {
       rowHeaderWidth,
@@ -476,6 +511,7 @@ export class GridUtils {
       allColumnXs,
       allColumnWidths,
       columnHeaderMaxDepth,
+      modelColumns,
     } = metrics;
     const { allowColumnResize, headerSeparatorHandleSize } = theme;
 
@@ -489,6 +525,7 @@ export class GridUtils {
 
     const gridX = x - rowHeaderWidth;
     const halfSeparatorSize = headerSeparatorHandleSize * 0.5;
+    const depth = GridUtils.getColumnHeaderDepthAtY(y, metrics);
 
     // Iterate through the floating columns first since they're on top
     let isPreviousColumnHidden = false;
@@ -507,7 +544,16 @@ export class GridUtils {
 
         const minX = midX - halfSeparatorSize;
         const maxX = midX + halfSeparatorSize;
-        if (minX <= gridX && gridX <= maxX) {
+        if (
+          minX <= gridX &&
+          gridX <= maxX &&
+          GridUtils.hasColumnSeparatorAtDepth(
+            model,
+            depth,
+            modelColumns.get(column),
+            modelColumns.get(column + 1)
+          )
+        ) {
           return column;
         }
 
@@ -538,7 +584,16 @@ export class GridUtils {
 
         const minX = midX - halfSeparatorSize;
         const maxX = midX + halfSeparatorSize;
-        if (minX <= gridX && gridX <= maxX) {
+        if (
+          minX <= gridX &&
+          gridX <= maxX &&
+          GridUtils.hasColumnSeparatorAtDepth(
+            model,
+            depth,
+            modelColumns.get(column),
+            modelColumns.get(column + 1)
+          )
+        ) {
           return column;
         }
 

packages/grid/src/mouse-handlers/GridColumnSeparatorMouseHandler.ts

@@ -28,7 +28,8 @@ class GridColumnSeparatorMouseHandler extends GridSeparatorMouseHandler {
       x,
       y,
       metrics,
-      theme
+      theme,
+      model
     );
 
     if (separatorIndex == null || depth == null) {