Is it possible to create a SDL3 wrap from an existing window without disturb existing behavior?

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Summary

A user reported an issue where wrapping an existing native window handle into an SDL3 window using SDL_CreateWindowWithProperties caused the existing mouse interaction behavior of the host application (DAW) to be disturbed. Specifically, hover states on native widgets stopped working. The root cause is that SDL3 attaches an internal WndProc (Windows) or X11 event handler to the provided window handle, intercepting events intended for the native UI framework. This is a fundamental limitation of how SDL manages window contexts; it requires ownership of the event loop for the window it creates.

Root Cause

The issue arises from the architectural design of SDL3 regarding window management:

  • Event Hooking: When SDL_CreateWindowWithProperties is called with SDL_PROP_WINDOW_CREATE_WIN32_HWND_POINTER (or the X11 equivalent), SDL does not merely “wrap” the handle in a passive container. It actively registers itself as the event handler for that specific window handle.
  • WndProc Replacement (Windows): On Windows, SDL typically subclasses the window or replaces the WndProc to intercept messages (mouse, keyboard, paint). This allows SDL to translate native OS events into SDL events (e.g., SDL_Event). However, this interception prevents the original application logic (the DAW’s UI framework) from receiving those messages.
  • Input Grabbing: By taking control of the event stream, SDL effectively “grabs” the input focus. The native widget tree underneath is starved of the messages required to calculate layout states like hover.

Why This Happens in Real Systems

This behavior is standard in graphics libraries that manage an event loop:

  • Single Source of Truth: Libraries like SDL, GLFW, or Qt generally assume they “own” the window they create. If they are given an existing window, they assume they are taking over that window’s rendering and input management.
  • OS-Level Limitations: You cannot easily have two distinct event pumps polling the same window handle simultaneously without complex message hooking (which SDL does not do for passive wrapping). The OS delivers a message (e.g., WM_MOUSEMOVE) to one destination only.
  • Abstraction Leak: While the goal was to use SDL for portability (avoiding platform-specific code), the requirement to preserve the host’s native UI behavior creates a conflict. SDL is designed to be the primary UI layer, not a secondary overlay on top of a third-party UI framework.

Real-World Impact

  • Host DAW Incompatibility: Audio plugins that embed their UI into a DAW host cannot use standard SDL window wrapping without breaking the host’s native skinning or widget interaction.
  • Broken User Experience: Users experience unresponsive buttons, missing hover effects, and glitchy text input, making the plugin feel “alien” or non-functional within the host.
  • Platform-Specific Workarounds: As noted by the user, engineers are forced to invert the control flow—creating an SDL window first and reparenting it into the host using platform-specific APIs (e.g., SetParent on Windows). This defeats the purpose of using SDL for window management in this specific integration scenario.

Example or Code

The user is attempting to use code similar to this, which triggers the issue:

// Hypothetical usage causing the issue
SDL_PropertiesID props = SDL_CreateProperties();
SDL_SetNumber(props, SDL_PROP_WINDOW_CREATE_WIN32_HWND_POINTER, (long long)native_hwnd);
SDL_Window* window = SDL_CreateWindowWithProperties(props);
// SDL now owns the event loop for 'native_hwnd', breaking the host UI

The correct approach for the user’s specific goal (reparenting) is actually the one they are currently using (reverse engineering), but here is the conceptual code for the correct separation of concerns:

// Correct approach: Create a child window, do not wrap the host
SDL_Window* plugin_window = SDL_CreateWindow("Plugin", 800, 600, SDL_WINDOW_HIDDEN);
// Use platform specific API to set parent (User's current working method)
SetParent((HWND)SDL_GetPointerProperty(SDL_GetWindowProperties(plugin_window), SDL_PROP_WINDOW_WIN32_HWND, 0), host_hwnd);
SDL_ShowWindow(plugin_window);

How Senior Engineers Fix It

Senior engineers recognize that wrapping an existing window handle into SDL while preserving the host’s UI behavior is technically impossible with SDL’s current architecture. The fix involves architectural decisions:

  1. Accept the Inversion of Control: Instead of trying to wrap the host, create a new, independent SDL window and reparent it into the host container using native OS APIs.
    • Benefit: The SDL window has its own event pump and does not interfere with the host’s native widgets.
    • Trade-off: You lose the “pure SDL” abstraction for window creation and must maintain small platform-specific snippets for reparenting.
  2. Use Offscreen Rendering (Advanced): If the goal is to render into the host window without a distinct SDL window, do not use SDL_CreateWindowWithProperties. Instead, retrieve the native device context (HDC) or Graphics context from the host, initialize SDL’s renderer with that context, and handle event pumping externally. However, SDL is not strictly designed for “headless” window contexts on all platforms, making this fragile.
  3. Framework Alignment: If the host is a DAW using a specific UI framework (e.g., JUCE, Qt), integrate the rendering logic into that framework’s draw loop rather than forcing SDL to own the window.

Why Juniors Miss It

Junior engineers often misinterpret the capability of window wrapping:

  • Assumption of Passive Wrapping: They assume SDL_CreateWindowWithProperties(HWND) simply attaches SDL rendering to an existing window handle without altering the window’s internal logic. They don’t realize SDL hooks the window to process input.
  • Underestimating Event Pumps: They may not understand that SDL is an event-driven library. For SDL to function (rendering, input, audio), it must poll and process messages. When it claims a window, it claims the messages associated with it.
  • Hoping for “Magic”: The desire to write “write once, run anywhere” code leads to hope that SDL can abstract away the complex parent-child window relationships required by DAW embedding standards. They often miss that DAW embedding is inherently platform-specific and requires native API calls for stability.