Index: ps/trunk/binaries/data/config/default.cfg
===================================================================
--- ps/trunk/binaries/data/config/default.cfg
+++ ps/trunk/binaries/data/config/default.cfg
@@ -31,6 +31,11 @@
 ; Hides a window border in the windowed mode.
 borderless.window = false
 
+; Constrain mouse in the fullscreen mode to a window boundaries.
+window.mousegrabinfullscreen = true
+; The same but for the window mode.
+window.mousegrabinwindowmode = false
+
 ; Show detailed tooltips (Unit stats)
 showdetailedtooltips = false
 
Index: ps/trunk/binaries/data/mods/public/gui/options/options.json
===================================================================
--- ps/trunk/binaries/data/mods/public/gui/options/options.json
+++ ps/trunk/binaries/data/mods/public/gui/options/options.json
@@ -176,6 +176,18 @@
 				"config": "gui.session.dragdelta",
 				"min": "1",
 				"max": "200"
+			},
+			{
+				"type": "boolean",
+				"label": "Mouse grab in fullscreen",
+				"tooltip": "Constrain mouse in the fullscreen mode to the window boundaries. It's used to avoid mouse going out of a display in case of multiple displays.",
+				"config": "window.mousegrabinfullscreen"
+			},
+			{
+				"type": "boolean",
+				"label": "Mouse grab in window mode",
+				"tooltip": "Constrain mouse in the window mode to the window boundaries.",
+				"config": "window.mousegrabinwindowmode"
 			}
 		]
 	},
Index: ps/trunk/source/ps/VideoMode.cpp
===================================================================
--- ps/trunk/source/ps/VideoMode.cpp
+++ ps/trunk/source/ps/VideoMode.cpp
@@ -515,7 +515,14 @@
 	SDL_GetWindowSize(m_Window, &m_CurrentW, &m_CurrentH);
 	m_CurrentBPP = bpp;
 
-	if (fullscreen)
+	// #545: we need to constrain the window in fullscreen mode to avoid mouse
+	// "falling out" of the window in case of multiple displays.
+	bool mouseGrabInFullscreen = true;
+	CFG_GET_VAL("window.mousegrabinfullscreen", mouseGrabInFullscreen);
+	bool mouseGrabInWindowMode = false;
+	CFG_GET_VAL("window.mousegrabinwindowmode", mouseGrabInWindowMode);
+
+	if (fullscreen ? mouseGrabInFullscreen : mouseGrabInWindowMode)
 		SDL_SetWindowGrab(m_Window, SDL_TRUE);
 	else
 		SDL_SetWindowGrab(m_Window, SDL_FALSE);