A drop‑in replacement for WinForms’ built‑in TabControl that removes the OS‑drawn border, paints its own buttons (owner‑draw), and keeps the page area truly transparent so the parent control can shine through.

• It’s part of Core Keepers Workshop and is released under the GNU GPL v3.
  • Also check out the CustomFormStyler (TitleBarManager).

1. Copy BorderlessTabControl.cs into your WinForms project.
2. Change the namespace if needed.
3. Re-build; the control will now appear in the designer toolbox.

ℹ️ No NuGet package (yet) – it’s a single self-contained file.

public partial class MainForm : Form
{
    public MainForm()
    {
        InitializeComponent();

        var tabs = new BorderlessTabControl
        {
            Dock = DockStyle.Fill, // Or wherever you like.
            Appearance = TabAppearance.FlatButtons
        };

        tabs.TabPages.Add("Home");
        tabs.TabPages.Add("Settings");

        Controls.Add(tabs);
    }
}

var page1 = Main_BorderlessTabControl.TabPages[0];
page1.Tag = new BorderlessTabControl.TabColorInfo(page1)
{
    TextFore = Color.White,     // Caption (text) color.
    FaceBack = Color.SteelBlue, // Button background.
    PageBack = Color.AliceBlue  // Optional: Page background.
};

using BorderlessTabControlExample; // Extension lives in the classes namespace.

Main_BorderlessTabControl.RecolorAllTabs(BorderlessTabControlExtensions.ThemeMode.Dark);

Main_BorderlessTabControl.NearestNeighborStretch = true;
page1.BackgroundImage = Properties.Resources.PixelBackdrop;

BorderlessTabControl intentionally keeps its surface area small—most behaviour is internal.
Anything you can do with the base TabControl still applies (add/remove pages, handle Selecting, etc.).

Main_BorderlessTabControl.RecolorAllTabs(BorderlessTabControlExtensions.ThemeMode.Dark);

• Background images – Set TabPage.BackgroundImage to a bitmap; it will fill the button surface (not the page) and honour NearestNeighborStretch.
• Custom bevel colour – Change the static BevelColor field to match your palette.
• Dock = Fill inset – The control intentionally shrinks its parent by 8 px on the first handle‑creation when it’s DockStyle.Fill. Call AdjustControlSize(true) later if you need to re‑apply the inset.

• CreateParams override – Clears WS_BORDER and WS_EX_CLIENTEDGE.
• WndProc hook – Catches TCM_ADJUSTRECT to expand the client rect → page area stays transparent; swallows WM_ERASEBKGND to avoid flicker.
• OnPaint – Draws each button manually, including pressed/selected states and separators for FlatButtons.
• Height fix – ReclaimNormalTabHeight() sends TCM_SETITEMSIZE after handle creation to normalise TabAppearance.Normal.
• Region syncing – Builds a custom Region each time size/layout changes so hit‑testing matches the irregular outline.

BorderlessTabControl.cs is licensed under GNU General Public License v3.0.
See the root LICENSE file for the full text.

Original author: RussDev7
Special thanks to the WinForms community for years of owner‑draw inspiration.

(Images showing the standard vs. borderless tab control.)

Found a bug or have an idea? Open an issue or pull request—contributions welcome!