Fix for autofit columns with autofilters

src/EPPlus/Core/AutofitHelper.cs

@@ -23,6 +23,8 @@ namespace OfficeOpenXml.Core
 {
     internal class AutofitHelper
     {
+        // Approximate width in pixels (at 96 DPI) of the autofilter dropdown arrow rendered by Excel.
+        private const double AutoFilterArrowWidthPixels = 15d;
         private ExcelRangeBase _range;
         ITextMeasurer _genericMeasurer = new GenericFontMetricsTextMeasurer();
         MeasurementFont _nonExistingFont = new MeasurementFont() { FontFamily = FontSize.NonExistingFont };
@@ -126,9 +128,19 @@ internal void AutofitColumn(double MinimumWidth, double MaximumWidth)
                 {
                     if (af.Collide(fromRow, col, toRow, col) != eAddressCollition.No)
                     {
-                        var cell = worksheet.Cells[af.Address];
+                        var cell = worksheet.Cells[af._fromRow, col];
                         var cellStyleId = styles.CellXfs[cell.StyleID];
                         currentMaxWidth = GetTextLength(cell, textLengthCache, styles, cellStyleId, normalSize, MaximumWidth, currentMaxWidth);
+                        // Reserve room for the autofilter dropdown arrow. The arrow is a fixed-size UI
+                        // element (~15px at 96 DPI), so it is added as a constant converted to column
+                        // width units (normalSize = width of the normal font's reference char in pixels).
+                        // It is intentionally not affected by AutofitScaleFactor, since the arrow's
+                        // pixel size does not shrink when the user requests tighter text margins.
+                        currentMaxWidth += AutoFilterArrowWidthPixels / normalSize;
+                        if (currentMaxWidth >= MaximumWidth)
+                        {
+                            currentMaxWidth = MaximumWidth;
+                        }
                     }
                 }
                 foreach (var cell in worksheet.Cells[fromRow, col, toRow, col])

src/EPPlusTest/Issues/WorksheetIssues.cs

@@ -160,6 +160,58 @@ public void i1314()
                 package.Dispose();
             }
         }
+
+        [TestMethod]
+        public void AutofitAutofilterTest()
+        {
+            using var package = OpenPackage("AutofitAutofilterTest.xlsx", true);
+
+            // Two sheets with identical data: one with an autofilter, one without.
+            // After autofit, the filtered columns should be wider than the unfiltered
+            // ones by the reserved width of the dropdown arrow.
+            var wsFilter = package.Workbook.Worksheets.Add("WithFilter");
+            var wsNoFilter = package.Workbook.Worksheets.Add("NoFilter");
+
+            foreach (var ws in new[] { wsFilter, wsNoFilter })
+            {
+                // Headers are the widest text in each column - the data below is deliberately
+                // shorter so the column width is driven by the header (+ the dropdown arrow
+                // on the filtered sheet).
+                ws.Cells["A1"].Value = "Department";
+                ws.Cells["B1"].Value = "Annual Budget";
+                ws.Cells["C1"].Value = "Region Name";
+
+                ws.Cells["A2"].Value = "Sales";
+                ws.Cells["B2"].Value = 1200;
+                ws.Cells["C2"].Value = "North";
+
+                ws.Cells["A3"].Value = "IT";
+                ws.Cells["B3"].Value = 980;
+                ws.Cells["C3"].Value = "West";
+
+                ws.Cells["A4"].Value = "HR";
+                ws.Cells["B4"].Value = 540;
+                ws.Cells["C4"].Value = "East";
+            }
+
+            // Only one sheet gets the autofilter.
+            wsFilter.Cells["A1:C4"].AutoFilter = true;
+
+            wsFilter.Cells["A1:C4"].AutoFitColumns();
+            wsNoFilter.Cells["A1:C4"].AutoFitColumns();
+
+            for (int col = 1; col <= 3; col++)
+            {
+                var filterWidth = wsFilter.Column(col).Width;
+                var noFilterWidth = wsNoFilter.Column(col).Width;
+                System.Diagnostics.Debug.WriteLine($"Column {col}: filter={filterWidth}, noFilter={noFilterWidth}");
+                Assert.IsTrue(filterWidth > noFilterWidth,
+                    $"Column {col}: filtered width ({filterWidth}) should be greater than unfiltered width ({noFilterWidth}).");
+            }
+
+            SaveAndCleanup(package);
+        }
+
         [TestMethod]
         public void i1317()
         {

src/EPPlusTest/WorkSheetTests.cs

@@ -2180,6 +2180,68 @@ public void AutoFitColumnTest()
             Assert.AreEqual(125d, ws.Columns[1].Width, 5d);
             SaveAndCleanup(p);
         }
+
+        [TestMethod]
+        public void AutofitAutofilterTest()
+        {
+            using var package = OpenTemplatePackage("AutoFitAutofilter.xlsx");
+            var ws = package.Workbook.Worksheets.Add("Sheet1");
+
+            // Headers are the widest text in each column - the data below is deliberately
+            // shorter so the column width is driven by the header + the autofilter dropdown arrow.
+            ws.Cells["A1"].Value = "Department";
+            ws.Cells["B1"].Value = "Annual Budget";
+            ws.Cells["C1"].Value = "Region Name";
+
+            // Data rows - all shorter than the headers above them.
+            ws.Cells["A2"].Value = "Sales";
+            ws.Cells["B2"].Value = 1200;
+            ws.Cells["C2"].Value = "North";
+
+            ws.Cells["A3"].Value = "IT";
+            ws.Cells["B3"].Value = 980;
+            ws.Cells["C3"].Value = "West";
+
+            ws.Cells["A4"].Value = "HR";
+            ws.Cells["B4"].Value = 540;
+            ws.Cells["C4"].Value = "East";
+
+            // Apply autofilter across the header row + data.
+            ws.Cells["A1:C4"].AutoFilter = true;
+
+            // Autofit the columns.
+            ws.Cells["A1:C4"].AutoFitColumns();
+
+            // Inspect what EPPlus actually produced for each column.
+            System.Diagnostics.Debug.WriteLine($"Column A (Department):    {ws.Column(1).Width}");
+            System.Diagnostics.Debug.WriteLine($"Column B (Annual Budget): {ws.Column(2).Width}");
+            System.Diagnostics.Debug.WriteLine($"Column C (Region Name):   {ws.Column(3).Width}");
+
+            // Save the workbook
+            SaveAndCleanup(package);
+        }
+
+        [TestMethod]
+        public void AutoFitColumnsWithAutoFilter()
+        {
+            var ws = _pck.Workbook.Worksheets.Add("AutofitAutoFilter");
+            ws.Cells["A1"].Value = "hour";
+            ws.Cells["B1"].Value = "minute";
+            ws.Cells["A2"].Value = 12;
+            ws.Cells["B2"].Value = 30;
+
+            ws.Cells["A1:B2"].AutoFilter = true;
+
+            ws.Cells["A1:B2"].AutoFitColumns();
+
+            // Without the fix, the AutoFilter header row range (A1:B1) is measured as a whole.
+            // Under the hood, worksheet.Cells["A1:B1"].TextForWidth evaluated to "System.Object[,]" (16 chars),
+            // which forced a minimum width of ~16.07 points.
+            // With the fix, the specific cell for each column in the AutoFilter is measured, 
+            // resulting in a narrow width matching "hour" / "minute".
+            Assert.IsTrue(ws.Column(1).Width < 12d, $"Column 1 width should be small but was {ws.Column(1).Width}");
+            Assert.IsTrue(ws.Column(2).Width < 12d, $"Column 2 width should be small but was {ws.Column(2).Width}");
+        }
         [TestMethod]
         public void CopyOverwrite()
         {