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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Windows.Forms;

namespace CustomWinFormsControls {
    /// <summary>
    /// Form with a <see cref="FormBorderStyle.None"/> which can be resized and moved if drag control is set.
    /// </summary>
    public class BorderlessResizableForm : Form {
        private FormDragControl dragControl;
        private Dictionary<Control, BorderWndProcFilter> borderFilters;

        private int _resizeBorderThickness = 5;

        /// <summary>
        /// The control which should be used to move the window around.
        /// </summary>
        [EditorBrowsable(EditorBrowsableState.Always)]
        [Browsable(true)]
        [DefaultValue(null)]
        [Category("Behaviour")]
        [Description("The control which should be used to move the window around.")]
        public Control DragTarget {
            get => this.dragControl?.Target;
            set {
                if (!(this.dragControl is null))
                    this.dragControl.Target = value;
            }
        }

        /// <summary>
        /// The thickness of the resize border on all sides of the window. The border is invisible. Setting it will not result in a padding.
        /// </summary>
        [EditorBrowsable(EditorBrowsableState.Always)]
        [Browsable(true)]
        [DefaultValue(null)]
        [Category("Behaviour")]
        [Description("The thickness of the resize border on all sides of the window. The border is invisible. Setting it will not result in a padding.")]
        public int ResizeBorderThickness {
            get => this._resizeBorderThickness;
            set {
                if (this._resizeBorderThickness == value)
                    return;

                this._resizeBorderThickness = value;

                foreach (BorderWndProcFilter filter in this.borderFilters.Values)
                    filter.BorderThinckness = value;
            }
        }

        /// <summary>
        /// Creates a new instance of the class <see cref="BorderlessResizableForm"/>
        /// </summary>
        public BorderlessResizableForm() {
            this.FormBorderStyle = FormBorderStyle.None;
            this.SetStyle(ControlStyles.ResizeRedraw, true);

            this.dragControl = new FormDragControl();
            this.borderFilters = new Dictionary<Control, BorderWndProcFilter>();
            this.ControlAdded += this.Control_ControlAdded;
            this.ControlRemoved += this.Control_ControlRemoved;
        }

        private void Control_ControlAdded(object sender, ControlEventArgs e) {
            this.ApplyFiltersRecursive(this, e.Control);
        }

        private void Control_ControlRemoved(object sender, ControlEventArgs e) {
            this.RemoveFiltersRecursive(e.Control);
        }

        private void ApplyFiltersRecursive(Control parent, Control control) {
            if (control is null)
                return;

            this.borderFilters[control] = new BorderWndProcFilter(parent, control, this.ResizeBorderThickness);

            // remove them in case they were already added.
            control.ControlAdded -= this.Control_ControlAdded;
            control.ControlRemoved -= this.Control_ControlRemoved;

            control.ControlAdded += this.Control_ControlAdded;
            control.ControlRemoved += this.Control_ControlRemoved;

            foreach (Control c in control.Controls)
                if (!(c is null))
                    this.ApplyFiltersRecursive(parent, c);
        }

        private void RemoveFiltersRecursive(Control control) {
            if (control is null)
                return;

            this.borderFilters.Remove(control);
            control.ControlAdded -= this.Control_ControlAdded;
            control.ControlRemoved -= this.Control_ControlRemoved;

            foreach (Control c in control.Controls)
                if (!(c is null))
                    this.RemoveFiltersRecursive(c);
        }

        protected override void WndProc(ref Message m) {
            if (m.Msg != BorderWndProcFilter.WM_NCHITTEST) {
                base.WndProc(ref m);
                return;
            }

            Point pos = this.PointToClient(new Point(m.LParam.ToInt32()));

            // if in top left corner
            if (pos.X <= this.ResizeBorderThickness && pos.Y <= this.ResizeBorderThickness) {
                m.Result = new IntPtr(BorderWndProcFilter.HTTOPLEFT);
                return;
            }

            // if in top right corner
            if (pos.X >= this.ClientSize.Width - this.ResizeBorderThickness && pos.Y <= this.ResizeBorderThickness) {
                m.Result = new IntPtr(BorderWndProcFilter.HTTOPRIGHT);
                return;
            }

            // if in bottom left corner
            if (pos.X <= this.ResizeBorderThickness && pos.Y >= this.ClientSize.Height - this.ResizeBorderThickness) {
                m.Result = new IntPtr(BorderWndProcFilter.HTBOTTOMLEFT);
                return;
            }

            // if in bottom right corner
            if (pos.X >= this.ClientSize.Width - this.ResizeBorderThickness && pos.Y >= this.ClientSize.Height - this.ResizeBorderThickness) {
                m.Result = new IntPtr(BorderWndProcFilter.HTBOTTOMRIGHT);
                return;
            }

            // if on the left
            if (pos.X <= this.ResizeBorderThickness) {
                m.Result = new IntPtr(BorderWndProcFilter.HTLEFT);
                return;
            }

            // if on top
            if (pos.Y <= this.ResizeBorderThickness) {
                m.Result = new IntPtr(BorderWndProcFilter.HTTOP);
                return;
            }

            // if on the right
            if (pos.X >= this.ClientSize.Width - this.ResizeBorderThickness) {
                m.Result = new IntPtr(BorderWndProcFilter.HTRIGHT);
                return;
            }

            // if on the bottom
            if (pos.Y >= this.ClientSize.Height - this.ResizeBorderThickness) {
                m.Result = new IntPtr(BorderWndProcFilter.HTBOTTOM);
                return;
            }

            base.WndProc(ref m);
        }
    }

    /// <summary>
    /// A filter for the WM_NCHITTEST message to ignore it when the mouse hovers the resize border.
    /// </summary>
    public class BorderWndProcFilter : NativeWindow {
        public const int WM_NCHITTEST = 0x0084;

        public const int HTCAPTION = 0x00000002;
        public const int HTTOP = 0x0000000C;
        public const int HTBOTTOM = 0x0000000F;
        public const int HTLEFT = 0x0000000A;
        public const int HTRIGHT = 0x0000000B;
        public const int HTTOPLEFT = 0x0000000D;
        public const int HTTOPRIGHT = 0x0000000E;
        public const int HTBOTTOMLEFT = 0x00000010;
        public const int HTBOTTOMRIGHT = 0x00000011;
        public const int HTTRANSPARENT = -1;

        public static List<Type> TypeBlacklist { get; set; } = new List<Type>();

        private Control parent;
        private Control child;

        public int BorderThinckness { get; set; }
        public bool ResizeBorderLeft { get; set; }
        public bool ResizeBorderRight { get; set; }
        public bool ResizeBorderTop { get; set; }
        public bool ResizeBorderBottom { get; set; }

        /// <summary>
        /// Creates a new instance of the class <see cref="BorderWndProcFilter"/>. Sets all borders to true.
        /// </summary>
        /// <param name="parent">The resizable control which should have a non blocked resize border.</param>
        /// <param name="child">The child control the filter should be applied to for not blocking the resize border of the parent control.</param>
        /// <param name="borderThinckness">The resize border thickness of the parent.</param>
        public BorderWndProcFilter(Control parent, Control child, int borderThinckness) {
            this.parent = parent;
            this.child = child;

            try {
                if (!BorderWndProcFilter.TypeBlacklist.Contains(child.GetType()))
                    this.AssignHandle(child.Handle);
            } catch (Exception) { }

            this.BorderThinckness = borderThinckness;

            this.ResizeBorderLeft = true;
            this.ResizeBorderRight = true;
            this.ResizeBorderTop = true;
            this.ResizeBorderBottom = true;
        }

        /// <summary>
        /// Creates a new instance of the class <see cref="BorderWndProcFilter"/>. Sets all borders to true.
        /// </summary>
        /// <param name="parent">The resizable control which should have a non blocked resize border.</param>
        /// <param name="child">The child control the filter should be applied to for not blocking the resize border of the parent control.</param>
        /// <param name="borderThinckness">The resize border thickness of the parent.</param>
        /// <param name="left">If true the parent has a left-side resize border which should not be blocked.</param>
        /// <param name="right">If true the parent has a right-side resize border which should not be blocked.</param>
        /// <param name="top">If true the parent has a top-side resize border which should not be blocked.</param>
        /// <param name="bottom">If true the parent has a bottom-side resize border which should not be blocked.</param>
        public BorderWndProcFilter(Control parent, Control child, int borderThinckness, bool left, bool right, bool top, bool bottom) {
            this.parent = parent;
            this.child = child;
            this.AssignHandle(child.Handle);
            this.BorderThinckness = borderThinckness;

            this.ResizeBorderLeft = left;
            this.ResizeBorderRight = right;
            this.ResizeBorderTop = top;
            this.ResizeBorderBottom = bottom;
        }

        protected override void WndProc(ref Message m) {
            if (m.Msg != WM_NCHITTEST) {
                base.WndProc(ref m);
                return;
            }

            if (this.parent is null) {
                base.WndProc(ref m);
                return;
            }

            Point pos = new Point(m.LParam.ToInt32());
            Point parentGlobalPos = this.parent is Form ? this.parent.Location : this.parent.PointToScreen(this.parent.Location);

            // if on the left
            if (pos.X <= parentGlobalPos.X + this.BorderThinckness && this.ResizeBorderLeft) {
                m.Result = new IntPtr(HTTRANSPARENT);
                return;
            }

            // if on top
            if (pos.Y <= parentGlobalPos.Y + this.BorderThinckness && this.ResizeBorderTop) {
                m.Result = new IntPtr(HTTRANSPARENT);
                return;
            }

            // if on the right
            if (pos.X >= parentGlobalPos.X + this.parent.Width - this.BorderThinckness && this.ResizeBorderRight) {
                m.Result = new IntPtr(HTTRANSPARENT);
                return;
            }

            // if on the bottom
            if (pos.Y >= parentGlobalPos.Y + this.parent.Height - this.BorderThinckness && this.ResizeBorderBottom) {
                m.Result = new IntPtr(HTTRANSPARENT);
                return;
            }

            base.WndProc(ref m);
        }
    }
}					

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if (LicenseManager.UsageMode == LicenseUsageMode.Designtime) {
    base.WndProc(ref m);
    return;
}