updating from review comments. DataFormats.None doesn't use IsUnused, so IsUnused should be removed a speculative code. Update ViewPort to not allow any more than one line-completions worth of empty bytes at the start.

Gen3Hex/ViewModel/DataFormats.cs

@@ -30,16 +30,10 @@ namespace HavenSoft.ViewModel.DataFormats {
    /// Basically everything is 'None' unless we have special information about it.
    /// </summary>
    public class None : IDataFormat {
-      public bool IsUnused { get; }
-      public None(byte value) => IsUnused = value == 0x00 || value == 0xFF;
+      public static None Instance { get; } = new None();
+      private None() { }
 
       public void Visit(IDataFormatVisitor visitor, byte data) => visitor.Visit(this, data);
-      public bool Equals(IDataFormat format) {
-         if (format is None that) {
-            return this.IsUnused == that.IsUnused;
-         }
-
-         return false;
-      }
+      public bool Equals(IDataFormat format) => format is None;
    }
 }

Gen3Hex/ViewModel/ViewPort.cs

@@ -36,7 +36,7 @@ namespace HavenSoft.Gen3Hex.ViewModel {
       public int Width {
          get => width;
          set {
-            if (Update(ref width, value)) {
+            if (Update(ref width, value) && width > 0 && height > 0) {
                UpdateScrollRange();
             }
          }
@@ -51,7 +51,7 @@ namespace HavenSoft.Gen3Hex.ViewModel {
       public int Height {
          get => height;
          set {
-            if (Update(ref height, value)) {
+            if (Update(ref height, value) && width > 0 && height > 0) {
                UpdateScrollRange();
             }
          }
@@ -111,7 +111,7 @@ namespace HavenSoft.Gen3Hex.ViewModel {
             if (index < 0 || index >= data.Length) return new HexElement { Format = Undefined.Instance };
 
             return new HexElement {
-               Format = new None(data[index]),
+               Format = None.Instance,
                Value = data[index],
             };
          }
@@ -130,12 +130,12 @@ namespace HavenSoft.Gen3Hex.ViewModel {
 
       private void UpdateScrollRange() {
          var lineCount = (int)Math.Ceiling((double)data.Length / width);
-         MinimumScroll = 1 - height;
+         MinimumScroll = 0;
          MaximumScroll = lineCount - 1;
          var newCurrentScroll = (int)Math.Ceiling((double)dataIndex / width);
 
          // screen size changes while scrolled above the data can make the data scroll completely out of view
-         while (newCurrentScroll <= -Height) {
+         while (newCurrentScroll < 0) {
             newCurrentScroll++;
             dataIndex += Width;
          }

HexTests/ViewPortTests.cs

@@ -32,6 +32,14 @@ namespace HavenSoft.HexTests {
          Assert.Equal(0, viewPort.MaximumScroll);
       }
 
+      [Fact]
+      public void ViewPortScrollStartsAtTopRow() {
+         var loadedFile = new LoadedFile("test", new byte[25]);
+         var viewPort = new ViewPort(loadedFile) { Width = 5, Height = 5 };
+
+         Assert.Equal(0, viewPort.ScrollValue);
+      }
+
       /// <summary>
       /// The scroll bar is in terms of lines.
       /// The viewport should not be able to scroll such that all the data is out of view.
@@ -41,7 +49,7 @@ namespace HavenSoft.HexTests {
          var loadedFile = new LoadedFile("test", new byte[25]);
          var viewPort = new ViewPort(loadedFile) { Width = 5, Height = 5 };
 
-         Assert.Equal(-4, viewPort.MinimumScroll);
+         Assert.Equal(0, viewPort.MinimumScroll);
          Assert.Equal(4, viewPort.MaximumScroll);
       }
 
@@ -52,7 +60,7 @@ namespace HavenSoft.HexTests {
 
          viewPort.ScrollValue = -10;
 
-         Assert.Equal(-4, viewPort.MinimumScroll);
+         Assert.Equal(0, viewPort.MinimumScroll);
       }
 
       [Fact]
@@ -79,6 +87,20 @@ namespace HavenSoft.HexTests {
          Assert.Equal(6, viewPort.MaximumScroll);
       }
 
+      [Fact]
+      public void ResizingCannotLeaveTotallyBlankLineAtTop() {
+         var loadedFile = new LoadedFile("test", new byte[25]);
+         var viewPort = new ViewPort(loadedFile) { Width = 5, Height = 5 };
+
+         viewPort.ScrollValue++;   // scroll down one line
+         viewPort.Width--;         // decrease the width so that there is data 2 lines above
+         viewPort.ScrollValue = 0; // scroll up to top
+         viewPort.Width--;         // decrease the width to make the top line totally blank
+
+         // expected: viewPort should auto-scroll here to make the top line full of data again
+         Assert.Equal(0, viewPort.ScrollValue);
+      }
+
       [Fact]
       public void RequestingOutOfRangeDataReturnsUnavailable() {
          var loadedFile = new LoadedFile("test", new byte[25]);