Top-level window size

Question

1. Why is the minimum size of a top-level window constrained to 2x2 pixels?
2. Is it possible to override this constraint so that I

Answer 1

The minimum size of a window is normally restricted to the smallest size that still allows the system menu and the caption buttons to be accessible by the user. Even for borderless windows, not exactly appropriate. You can make it smaller by explicitly setting the Size property after the window is created, normally the Load event handler is your first opportunity. Or OnHandleCreated().

But yeah, Windows still restricts it to 2x2, the "why" is elusive. Surely what you really want to know is how to work around it. You do so by intercepting the WM_GETMINMAXINFO message and lowering the minimum track size. A sample form class that demonstrates the approach:

Imports System.Runtime.InteropServices

Public Class Form1
    Protected Overrides Sub WndProc(ByRef m As Message)
        MyBase.WndProc(m)
        If m.Msg = WM_GETMINMAXINFO Then
            Dim mmi = DirectCast(Marshal.PtrToStructure(m.LParam, GetType(MINMAXINFO)), MINMAXINFO)
            mmi.ptMinTrackSize = New Point(0, 0)
            Marshal.StructureToPtr(mmi, m.LParam, False)
        End If
    End Sub

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Me.Size = New Size(0, 0)
        MsgBox(Me.Size.ToString())
    End Sub

    Private Const WM_GETMINMAXINFO As Integer = &h24
    Private Structure MINMAXINFO
        Public ptReserved As Point
        Public ptMaxSize As Point
        Public ptMaxPosition As Point
        Public ptMinTrackSize As Point
        Public ptMaxTrackSize As Point
    End Structure
End Class

Tested on Windows 8.1. I can't promise that it will work on all Windows versions. It should.