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Borderless Fullscreen

The Interposer can convert game windows to borderless fullscreen when they are created. The game runs at its intended resolution but without a title bar, borders, or resize handles, and the window is centered on the monitor it appears on.

This feature is opt-in and must be enabled in .interposer/Config.yml.

What It Does​

When a game creates a top-level window, the Interposer:

  1. Strips the window border styles (WS_THICKFRAME, WS_DLGFRAME, WS_BORDER)
  2. Strips the extended border styles (WS_EX_DLGMODALFRAME, WS_EX_WINDOWEDGE, WS_EX_CLIENTEDGE, WS_EX_STATICEDGE)
  3. Centers the window on the monitor it was created on

If the game later tries to re-apply border styles — which some engines do after window creation — those attempts are also intercepted and silently discarded.

Why This Is Useful​

Many older games only support exclusive fullscreen mode or windowed mode with a border. Exclusive fullscreen causes problems on modern multi-monitor setups and prevents Alt-Tab from working cleanly. Borderless fullscreen gives the game a full-screen appearance while behaving like a normal window to the operating system.

Common benefits:

  • Clean Alt-Tab switching without the display mode flickering
  • Window shares the desktop with other applications (useful for streaming or overlay software)
  • Consistent behavior across different screen resolutions and DPI settings

Window Detection​

Not every window a game creates is treated as the main game window. The Interposer applies the borderless treatment only to windows that look like a primary game window:

  • No parent window (top-level only — dialogs, splash screens, and child windows are left untouched)
  • Positive width and height
  • Created with WS_OVERLAPPEDWINDOW or WS_POPUP style bits

Splash screens and other short-lived windows are typically dismissed before the main window appears, so they are generally unaffected in practice.

Hooked Functions​

FunctionPurpose
CreateWindowExW / CreateWindowExAStrip border styles at creation time; center the window.
SetWindowPosPrevent the game from moving or resizing the window after creation.
SetWindowLongW / SetWindowLongAPrevent the game from re-applying border styles after creation.

Enabling Borderless Fullscreen​

Set Borderless: true in the Window section of .interposer/Config.yml:

Window:
  Borderless: true

The default is false — if the Window section is absent the game window is left untouched.