Fix macOS momentum scrolling: apply SDL hints before first video init

SDL_HINT_MAC_SCROLL_MOMENTUM is consumed exactly once by the Cocoa
backend's registerUserDefaults during the first SDL_InitSubSystem(
SDL_INIT_VIDEO), which registers AppleMomentumScrollSupported. The splash
screen brings video up before init_sdl() set the hint, so SDL read the
unset hint (default off) and momentum-phase scroll events were never
delivered. The later hint in init_sdl() was dead because video was
already up (ref-counted), so registerUserDefaults never re-ran.

Extract the hints into set_sdl_hints() (once-guarded) and call it before
the first video init from every entry point that can reach it first: the
splash screen, init_sdl(), and OSMessageBoxSDL (early fatal dialogs route
through Cocoa_RegisterApp too).

Also restores SDL_HINT_MAC_PRESS_AND_HOLD, which fed the same
registerUserDefaults and was being applied just as late.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: RyeMutt

indra/llwindow/llsdl.cpp

@@ -72,37 +72,17 @@ void sdl_logger(void *userdata, int category, SDL_LogPriority priority, const ch
 
 #endif
 
-void init_sdl(const std::string& app_name)
+void set_sdl_hints()
 {
-#ifndef LL_SDL_WINDOW
-    if (!gSDLMainHandled)
-    {
-        SDL_SetMainReady();
-    }
-
-    SDL_SetLogOutputFunction(&sdl_logger, nullptr);
-
-    const int c_sdl_version = SDL_VERSION;
-    LL_INFOS() << "Compiled against SDL "
-               << SDL_VERSIONNUM_MAJOR(c_sdl_version) << "."
-               << SDL_VERSIONNUM_MINOR(c_sdl_version) << "."
-               << SDL_VERSIONNUM_MICRO(c_sdl_version) << LL_ENDL;
-    const int r_sdl_version = SDL_GetVersion();
-    LL_INFOS() << "Running with SDL "
-               << SDL_VERSIONNUM_MAJOR(r_sdl_version) << "."
-               << SDL_VERSIONNUM_MINOR(r_sdl_version) << "."
-               << SDL_VERSIONNUM_MICRO(r_sdl_version) << LL_ENDL;
-#endif
-
-#if LL_WINDOWS && defined(LL_SDL_WINDOW)
-    Uint32 style = 0;
-#if defined(CS_BYTEALIGNCLIENT) && defined(CS_OWNDC)
-    style = (CS_BYTEALIGNCLIENT | CS_OWNDC);
-#endif
-    SDL_RegisterApp(app_name.c_str(), style, nullptr);
-#endif
-
 #if LL_SDL_WINDOW
+    // Run once. SDL_SetHint is itself idempotent, but the registerUserDefaults
+    // hints only take effect on the *first* video init, so re-setting them after
+    // that point would be pointless — bail early to make the intent explicit.
+    static bool sHintsSet = false;
+    if (sHintsSet)
+        return;
+    sHintsSet = true;
+
     std::initializer_list<std::tuple< char const*, char const * > > hintList =
             {
                     // Don't ask the compositor to bypass us in fullscreen —
@@ -163,6 +143,42 @@ void init_sdl(const std::string& app_name)
         SDL_SetHint(std::get<0>(hint), std::get<1>(hint));
     }
 #endif
+}
+
+void init_sdl(const std::string& app_name)
+{
+#ifndef LL_SDL_WINDOW
+    if (!gSDLMainHandled)
+    {
+        SDL_SetMainReady();
+    }
+
+    SDL_SetLogOutputFunction(&sdl_logger, nullptr);
+
+    const int c_sdl_version = SDL_VERSION;
+    LL_INFOS() << "Compiled against SDL "
+               << SDL_VERSIONNUM_MAJOR(c_sdl_version) << "."
+               << SDL_VERSIONNUM_MINOR(c_sdl_version) << "."
+               << SDL_VERSIONNUM_MICRO(c_sdl_version) << LL_ENDL;
+    const int r_sdl_version = SDL_GetVersion();
+    LL_INFOS() << "Running with SDL "
+               << SDL_VERSIONNUM_MAJOR(r_sdl_version) << "."
+               << SDL_VERSIONNUM_MINOR(r_sdl_version) << "."
+               << SDL_VERSIONNUM_MICRO(r_sdl_version) << LL_ENDL;
+#endif
+
+#if LL_WINDOWS && defined(LL_SDL_WINDOW)
+    Uint32 style = 0;
+#if defined(CS_BYTEALIGNCLIENT) && defined(CS_OWNDC)
+    style = (CS_BYTEALIGNCLIENT | CS_OWNDC);
+#endif
+    SDL_RegisterApp(app_name.c_str(), style, nullptr);
+#endif
+
+    // Hints must be in place before the first video init. The splash screen
+    // also calls this before its own SDL_InitSubSystem(SDL_INIT_VIDEO); the
+    // once-guard inside makes a second call here a no-op.
+    set_sdl_hints();
 
     // SDL_INIT_VIDEO is the only subsystem the viewer actually uses through
     // SDL3. Joystick / gamepad input goes through libndof (llviewerjoystick),

indra/llwindow/llsdl.h

@@ -30,5 +30,11 @@
 
 extern bool gSDLMainHandled;
 
+// Apply our SDL hints. Must run before the *first* SDL_InitSubSystem(SDL_INIT_VIDEO)
+// of the process: some hints (notably SDL_HINT_MAC_SCROLL_MOMENTUM and
+// SDL_HINT_MAC_PRESS_AND_HOLD) are consumed by SDL's Cocoa registerUserDefaults,
+// which runs once during the first video init and is never revisited. The splash
+// screen brings video up before init_sdl(), so it calls this first too. Idempotent.
+void set_sdl_hints();
 void init_sdl(const std::string& app_name);
 void quit_sdl();

indra/llwindow/llwindowsdl.cpp

@@ -3548,6 +3548,14 @@ void LLSplashScreenSDL::showImpl()
     // subsystem may not be up yet. SDL_InitSubSystem is reference-counted, so
     // initialising it here is safe; hideImpl() releases exactly this reference,
     // leaving the count the main window's own init_sdl() established.
+    //
+    // This is the process's *first* video init, so the SDL hints must already
+    // be set: the Cocoa backend's registerUserDefaults reads them once here and
+    // never again (e.g. SDL_HINT_MAC_SCROLL_MOMENTUM → AppleMomentumScrollSupported).
+    // Setting them only in init_sdl() would be too late and silently disable
+    // macOS momentum scrolling. set_sdl_hints() is idempotent.
+    set_sdl_hints();
+
     if (!SDL_InitSubSystem(SDL_INIT_VIDEO))
     {
         LL_WARNS() << "Splash: SDL_InitSubSystem(VIDEO) failed: " << SDL_GetError() << LL_ENDL;
@@ -3740,6 +3748,13 @@ void LLSplashScreenSDL::hideImpl()
 
 S32 OSMessageBoxSDL(const std::string& text, const std::string& caption, U32 type)
 {
+    // A fatal-error dialog can fire before the splash or main window has brought
+    // video up. On macOS SDL_ShowMessageBox routes through Cocoa_RegisterApp →
+    // registerUserDefaults, which reads our hints exactly once. Make sure they're
+    // set first so an early dialog doesn't lock in the wrong defaults (e.g.
+    // momentum scrolling). Idempotent if hints are already applied.
+    set_sdl_hints();
+
     // Use the main viewer window as the message box's parent so it is modal
     // to the viewer and stacks correctly above fullscreen on compositors that
     // place dialogs relative to their parent window.