Hardcode fullscreen mode OFF and hide the UI for it on mac until that's fixed. For now, users can use the native cocoa fullscreen mode

indra/newview/llappviewer.cpp

@@ -3553,7 +3553,15 @@ bool LLAppViewer::initWindow()
 		.height(gSavedSettings.getU32("WindowHeight"))
 		.min_width(gSavedSettings.getU32("MinWindowWidth"))
 		.min_height(gSavedSettings.getU32("MinWindowHeight"))
+/// <FS:CR> Since the 3.6.5 merge, setting fullscreen does terrible bad things on macs like opening
+/// all floaters and menus off the left side of the screen. Let's not do that right now...
+/// Hardcoding full screen OFF until it's fixed. On 10.7+ we have native full screen support anyway.
+#ifndef LL_DARWIN
 		.fullscreen(gSavedSettings.getBOOL("FullScreen"))
+#else // !LL_DARWIN
+		.fullscreen(false)
+#endif LL_DARWIN
+// </FS:CR>
 		.ignore_pixel_depth(ignorePixelDepth);
 
 	gViewerWindow = new LLViewerWindow(window_params);

indra/newview/llfloaterpreference.cpp

@@ -2744,6 +2744,11 @@ static LLRegisterPanelClassWrapper<LLPanelPreferencePrivacy> t_pref_privacy("pan
 BOOL LLPanelPreferenceGraphics::postBuild()
 {
 	mButtonApply=findChild<LLButton>("Apply");
+// <FS:CR> Hide this until we have fullscreen mode functional on OSX again
+#ifdef LL_DARWIN
+	getChild<LLCheckBoxCtrl>("Fullscreen Mode")->setVisible(FALSE);
+#endif // LL_DARWIN
+// </FS:CR>
 
 	return LLPanelPreference::postBuild();
 }