Added a relative mouse mode that uses mouse warping instead of raw input.

To enable this, set the environment variable SDL_MOUSE_RELATIVE_MODE_WARP to "1"

When mouse relative mode is disabled, put the cursor back where the application expects it to be, instead of where it was when relative mode was enabled.

src/events/SDL_mouse.c

@@ -23,6 +23,7 @@
 /* General mouse handling code for SDL */
 
 #include "SDL_assert.h"
+#include "SDL_hints.h"
 #include "SDL_timer.h"
 #include "SDL_events.h"
 #include "SDL_events_c.h"
@@ -205,12 +206,24 @@ SDL_PrivateSendMouseMotion(SDL_Window * window, SDL_MouseID mouseID, int relativ
     int yrel;
     int x_max = 0, y_max = 0;
 
-    /* relative motion is calculated regarding the system cursor last position */
+    if (mouse->relative_mode_warp) {
+        int center_x = 0, center_y = 0;
+        SDL_GetWindowSize(window, &center_x, &center_y);
+        center_x /= 2;
+        center_y /= 2;
+        if (x == center_x && y == center_y) {
+            mouse->last_x = center_x;
+            mouse->last_y = center_y;
+            return 0;
+        }
+        SDL_WarpMouseInWindow(window, center_x, center_y);
+    }
+
     if (relative) {
         xrel = x;
         yrel = y;
-        x = (mouse->last_x + x);
-        y = (mouse->last_y + y);
+        x = (mouse->last_x + xrel);
+        y = (mouse->last_y + yrel);
     } else {
         xrel = x - mouse->last_x;
         yrel = y - mouse->last_y;
@@ -225,7 +238,7 @@ SDL_PrivateSendMouseMotion(SDL_Window * window, SDL_MouseID mouseID, int relativ
     }
 
     /* Update internal mouse coordinates */
-    if (mouse->relative_mode == SDL_FALSE) {
+    if (!mouse->relative_mode) {
         mouse->x = x;
         mouse->y = y;
     } else {
@@ -481,21 +494,36 @@ SDL_WarpMouseInWindow(SDL_Window * window, int x, int y)
     }
 }
 
+static SDL_bool
+ShouldUseRelativeModeWarp(SDL_Mouse *mouse)
+{
+    const char *hint;
+
+    if (!mouse->SetRelativeMouseMode) {
+        return SDL_TRUE;
+    }
+
+    hint = SDL_GetHint(SDL_HINT_MOUSE_RELATIVE_MODE_WARP);
+    if (hint) {
+        if (*hint == '0') {
+            return SDL_FALSE;
+        } else {
+            return SDL_TRUE;
+        }
+    }
+    return SDL_FALSE;
+}
+
 int
 SDL_SetRelativeMouseMode(SDL_bool enabled)
 {
     SDL_Mouse *mouse = SDL_GetMouse();
     SDL_Window *focusWindow = SDL_GetKeyboardFocus();
-    int original_x = mouse->x, original_y = mouse->y;
 
     if (enabled == mouse->relative_mode) {
         return 0;
     }
 
-    if (!mouse->SetRelativeMouseMode) {
-        return SDL_Unsupported();
-    }
-
     if (enabled && focusWindow) {
         /* Center it in the focused window to prevent clicks from going through
          * to background windows.
@@ -504,23 +532,26 @@ SDL_SetRelativeMouseMode(SDL_bool enabled)
         SDL_WarpMouseInWindow(focusWindow, focusWindow->w/2, focusWindow->h/2);
     }
 
-    if (mouse->SetRelativeMouseMode(enabled) < 0) {
+    /* Set the relative mode */
+    if (!enabled && mouse->relative_mode_warp) {
+        mouse->relative_mode_warp = SDL_FALSE;
+    } else if (enabled && ShouldUseRelativeModeWarp(mouse)) {
+        mouse->relative_mode_warp = SDL_TRUE;
+    } else if (mouse->SetRelativeMouseMode(enabled) < 0) {
         return -1;
     }
-
-    /* Set the relative mode */
     mouse->relative_mode = enabled;
 
-    if (enabled) {
-        /* Save the expected mouse position */
-        mouse->original_x = original_x;
-        mouse->original_y = original_y;
-    } else if (mouse->focus) {
-        /* Restore the expected mouse position */
-        SDL_WarpMouseInWindow(mouse->focus, mouse->original_x, mouse->original_y);
+    if (mouse->focus) {
+        SDL_UpdateWindowGrab(mouse->focus);
+
+        /* Put the cursor back to where the application expects it */
+        if (!enabled) {
+            SDL_WarpMouseInWindow(mouse->focus, mouse->x, mouse->y);
+        }
     }
 
-    /* Flush pending mouse motion */
+    /* Flush pending mouse motion - ideally we would pump events, but that's not always safe */
     SDL_FlushEvent(SDL_MOUSEMOTION);
 
     /* Update cursor visibility */

src/events/SDL_mouse_c.h

@@ -73,8 +73,7 @@ typedef struct
     int last_x, last_y;         /* the last reported x and y coordinates */
     Uint32 buttonstate;
     SDL_bool relative_mode;
-    /* the x and y coordinates when relative mode was activated */
-    int original_x, original_y;
+    SDL_bool relative_mode_warp;
 
     /* Data for double-click tracking */
     int num_clickstates;