[EuiDataGrid] improve height calculation

src/components/datagrid/body/data_grid_body.tsx

@@ -573,11 +573,33 @@ export const EuiDataGridBody: FunctionComponent<EuiDataGridBodyProps> = (
     }
   }, [getRowHeight]);
 
+  let knownHeight = 0;
+  let knownRowCount = 0;
+  for (let i = startRow; i < endRow; i++) {
+    const correctRowIndex = getCorrectRowIndex(i);
+    const rowHeightOption = rowHeightUtils.getRowHeightOption(
+      correctRowIndex,
+      rowHeightsOptions
+    );
+    if (rowHeightOption) {
+      knownRowCount++;
+      knownHeight += rowHeightUtils.getCalculatedHeight(
+        rowHeightOption,
+        defaultHeight, // minRowHeight,
+        correctRowIndex,
+        rowHeightUtils.isRowHeightOverride(correctRowIndex, rowHeightsOptions)
+      );
+    }
+  }
+
   const rowCountToAffordFor = pagination
     ? pagination.pageSize
     : visibleRowIndices.length;
   const unconstrainedHeight =
-    defaultHeight * rowCountToAffordFor + headerRowHeight + footerRowHeight;
+    defaultHeight * (rowCountToAffordFor - knownRowCount) +
+    knownHeight +
+    headerRowHeight +
+    footerRowHeight;
 
   // unable to determine this until the container's size is known anyway
   const unconstrainedWidth = 0;

src/components/datagrid/data_grid.spec.tsx

@@ -51,6 +51,35 @@ describe('EuiDataGrid', () => {
         .should('have.lengthOf', 0);
     });
   });
+
+  describe('height calculation', async () => {
+    it('computes a new unconstrained height when switching to auto height', () => {
+      const renderCellValue: EuiDataGridProps['renderCellValue'] = ({
+        rowIndex,
+        columnId,
+      }) => (
+        <>
+          row {rowIndex}
+          <br />
+          column {columnId}
+        </>
+      );
+
+      mount(<EuiDataGrid {...baseProps} renderCellValue={renderCellValue} />);
+
+      getGridData();
+      cy.get('[data-test-subj=euiDataGridBody]')
+        .invoke('outerHeight')
+        .then((firstHeight) => {
+          cy.get('[data-test-subj=dataGridDisplaySelectorPopover]').click();
+          cy.get('[data-text="Auto fit"]').click();
+
+          cy.get('[data-test-subj=euiDataGridBody]')
+            .invoke('outerHeight')
+            .should('be.greaterThan', firstHeight);
+        });
+    });
+  });
 });
 
 function getGridData() {