Winforms (Horizontal) Scrollbar Unexplained Behavior - winforms

Question

When I resize my form with the resize handle on the bottom right corner of the form, it fires the sizechanged event of my user control. Inside the user control I have code to set the maximum and value of the horizontal scroll bar.
If I resize to the right, the code works as expected. When I resize to the left, the scrolling thumb gets really big and scrolling does not work as expected. It's as if the Maximum got set to a low number, but my Debug.WriteLine shows that this is not the case. Actually, as I slowly resize the form to be narrower, it quickly toggles between doing it right and doing it wrong.
I deal with scroll bars rarely and when I do they are always a pain. Is there a ScrollBar guru who knows why this is happening? I googled it and searched SO too, but I really don't know what to search on.
Here is my code. The relevant part is what is called from the sizechanged event handler, which is almost to the bottom of the code.
Imports System.Reflection
Public Class Grid
Public Sub New()
' This call is required by the designer.
InitializeComponent()
' Add any initialization after the InitializeComponent() call.
SetStyle(ControlStyles.AllPaintingInWmPaint Or ControlStyles.UserPaint Or ControlStyles.DoubleBuffer, True) 'Or ControlStyles.ResizeRedraw
End Sub
Private _FirstVisibleRow As Row
Private Property FirstVisibleRow As Row
Get
If _FirstVisibleRow Is Nothing And Rows.Any Then
_FirstVisibleRow = Rows.First
End If
Return _FirstVisibleRow
End Get
Set(value As Row)
_FirstVisibleRow = value
LastVisibleRow = GetLastVisibleRow()
End Set
End Property
Private _HeaderRow As Row
Public Property HeaderRow As Row
Get
If _HeaderRow Is Nothing Then
_HeaderRow = New Row(Me, RowHeight)
For Each Column As Column In Columns
_HeaderRow.Cells.Add(New Cell(Column, _HeaderRow))
Next
End If
Return _HeaderRow
End Get
Private Set(value As Row)
_HeaderRow = value
End Set
End Property
Private _LastVisibleRow As Row
Private Property LastVisibleRow As Row
Get
If _LastVisibleRow Is Nothing Then
_LastVisibleRow = GetLastVisibleRow()
End If
Return _LastVisibleRow
End Get
Set(value As Row)
_LastVisibleRow = value
End Set
End Property
Private _TotalColumnWidth As Integer
Friend Property TotalColumnWidth As Integer
Get
If _TotalColumnWidth = Nothing Then
_TotalColumnWidth = GetTotalColumnWidth()
End If
Return _TotalColumnWidth
End Get
Set(value As Integer)
_TotalColumnWidth = value
SetScrollBarVisibility()
End Set
End Property
Private _TotalRowHeight As Integer
Friend Property TotalRowHeight As Integer
Get
If _TotalRowHeight = Nothing Then
_TotalRowHeight = GetTotalRowHeight()
End If
Return _TotalRowHeight
End Get
Set(value As Integer)
_TotalRowHeight = value
SetScrollBarVisibility()
End Set
End Property
Private _VisibleGridSize As Size
Private Property VisibleGridSize As Size
Get
If _VisibleGridSize = Nothing Then
_VisibleGridSize = GetVisibleGridSize()
End If
Return _VisibleGridSize
End Get
Set(value As Size)
_VisibleGridSize = value
VisibleRowCount = GetVisibleRowCount()
SetScrollBarVisibility()
End Set
End Property
Private Sub SetScrollBarVisibility()
VScrollBar1.Bounds = New Rectangle(Width - VScrollBar1.Width, 0, VScrollBar1.Width, Height - IIf(HScrollBar1.Visible, HScrollBar1.Height, 0))
HScrollBar1.Bounds = New Rectangle(0, Height - HScrollBar1.Height, Width - IIf(VScrollBar1.Visible, VScrollBar1.Width, 0), HScrollBar1.Height)
VScrollBar1.Maximum = Math.Max(0, TotalRowHeight - Height - IIf(HScrollBar1.Visible, HScrollBar1.Height, 0))
HScrollBar1.Maximum = Math.Max(0, TotalColumnWidth - Width + IIf(VScrollBar1.Visible, VScrollBar1.Width, 0))
HScrollBar1.Value = 0
VScrollBar1.Visible = TotalRowHeight > VisibleGridSize.Height
HScrollBar1.Visible = TotalColumnWidth > VisibleGridSize.Width
Debug.WriteLine(String.Format("HScrollBar1.Minimum {0}, HScrollBar1.Maximum {1}, HScrollBar1.Value {2}", HScrollBar1.Minimum, HScrollBar1.Maximum, HScrollBar1.Value))
End Sub
Private _VisibleRowCount As Integer
Private Property VisibleRowCount As Integer
Get
If _VisibleRowCount = 0 Then
_VisibleRowCount = GetVisibleRowCount()
End If
Return _VisibleRowCount
End Get
Set(value As Integer)
_VisibleRowCount = value
LastVisibleRow = GetLastVisibleRow()
PageHeight = GetPageHeight()
End Set
End Property
Private Function GetLastVisibleRow() As Row
If Not Rows.Any Then Return Nothing
Return Rows(Math.Min(FirstVisibleRow.Index + VisibleRowCount - 1, Rows.Count - 1))
End Function
Private Function GetPageHeight() As Integer
Return RowHeight * GetVisibleRowCount()
End Function
Private Function GetRowHeight() As Integer
Return TextRenderer.MeasureText("X", Font).Height + 6
End Function
Private Function GetVisibleGridSize() As Size
Return New Size(Width - IIf(VScrollBar1.Visible, VScrollBar1.Width, 0),
Height - HeaderRow.Height - IIf(HScrollBar1.Visible, HScrollBar1.Height, 0)) 'don't count header row or horiz scroll bar
End Function
Private Function GetVisibleRowCount() As Integer
Return Math.Ceiling(VisibleGridSize.Height / RowHeight)
End Function
Public Shadows Sub Refresh()
ClearSelection()
_HeaderRow = Nothing
_Rows = Nothing
AutoSizeColumns()
TotalRowHeight = GetTotalRowHeight()
Invalidate()
End Sub
Friend Function GetTotalColumnWidth() As Integer
Return Columns.Select(Function(x) x.Width).Aggregate(0, Function(x, y) x + y)
End Function
Friend Function GetTotalRowHeight() As Integer
Return Rows.Select(Function(x) x.Height).Aggregate(0, Function(x, y) x + y)
End Function
Private Function VisibleRows() As List(Of Row)
Return Rows.Where(Function(x) x.Index >= FirstVisibleRow.Index AndAlso x.Index <= LastVisibleRow.Index).ToList
End Function
Private Sub Grid_Paint(sender As Object, e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint
e.Graphics.Clear(BackColor)
'e.ClipRectangle
Dim Left As Integer = -HScrollBar1.Value
For Each Column As Column In Columns
Left = Column.Draw(e.Graphics, Left)
Next
Dim Top As Integer = HeaderRow.Draw(e.Graphics)
For Each Row As Row In VisibleRows()
Top = Row.Draw(e.Graphics, Top)
Next
End Sub
Private Sub Grid_Scroll(sender As Object, e As System.Windows.Forms.ScrollEventArgs) Handles Me.Scroll
If e.ScrollOrientation = ScrollOrientation.VerticalScroll Then
Select Case e.Type
Case ScrollEventType.First
FirstVisibleRow = Rows.First
Case ScrollEventType.Last
FirstVisibleRow = Rows(Rows.Last.Index - VisibleRowCount + 1)
Case ScrollEventType.SmallDecrement
FirstVisibleRow = Rows(Math.Max(FirstVisibleRow.Index - 1, 0))
Case ScrollEventType.SmallIncrement
FirstVisibleRow = Rows(Math.Min(FirstVisibleRow.Index + 1, Rows.Last.Index - VisibleRowCount + 1))
Case ScrollEventType.LargeDecrement
FirstVisibleRow = Rows(Math.Max(FirstVisibleRow.Index - VisibleRowCount, 0))
Case ScrollEventType.LargeIncrement
FirstVisibleRow = Rows(Math.Min(LastVisibleRow.Index, Rows.Last.Index - VisibleRowCount + 1))
End Select
End If
Invalidate()
End Sub
Private Sub Grid_SizeChanged(sender As Object, e As System.EventArgs) Handles Me.SizeChanged
VisibleGridSize = GetVisibleGridSize()
End Sub
Private Sub HScrollBar1_Scroll(sender As Object, e As System.Windows.Forms.ScrollEventArgs) Handles HScrollBar1.Scroll
Invalidate()
End Sub
End Class

Answer 1

The AutoScroll property of my UserControl was getting set to True by a parent control. Once I set it back to False in the designer, my code works as expected 100% of the time.