[client] input: stop absolute cursor feedback

X11's legacy guest-position synchronization warps the host cursor to
delayed guest updates. With absolute input this feeds old positions back
through cooked motion and pulls the cursor away from window edges.

Skip display-server guest synchronization when the active input is
absolute. Relative input and explicit uncapture alignment are unchanged.

client/src/core.c

@@ -642,6 +642,11 @@ void core_stopFrameThread(void)
 
 void core_handleGuestMouseUpdate(void)
 {
+  /* The local cursor is authoritative with absolute input. Sending delayed
+   * guest positions back to the display server creates a warp feedback loop. */
+  if (lgInput_supports(LG_INPUT_SUPPORT_MOUSE_ABSOLUTE))
+    return;
+
   struct DoublePoint localPos;
   if (!util_guestCurToLocal(&localPos))
   {

client/tests/CMakeLists.txt

@@ -120,6 +120,7 @@ set(MOUSE_CASES
   surface-exit
   exit-immediate
   exit-guest
+  absolute-sync
   exit-reentry
   view-immediate
   capture-pending

client/tests/mouse_test.c

@@ -262,9 +262,8 @@ void app_mouseTrace(const char * file, unsigned int line,
 
 static bool inputSupports(void * opaque, LG_InputSupport support)
 {
-  (void)opaque;
+  return support == LG_INPUT_SUPPORT_MOUSE_ABSOLUTE &&
-  (void)support;
+    opaque && *(const bool *)opaque;
-  return false;
 }
 
 static bool inputKey(void * opaque, int key)
@@ -281,6 +280,17 @@ static bool inputMouseMotion(void * opaque, int32_t x, int32_t y)
   return true;
 }
 
+static bool inputMousePosition(void * opaque, uint32_t x, uint32_t y,
+    uint32_t width, uint32_t height)
+{
+  (void)opaque;
+  (void)x;
+  (void)y;
+  (void)width;
+  (void)height;
+  return true;
+}
+
 static bool inputMouseButton(void * opaque, unsigned int button)
 {
   (void)opaque;
@@ -294,6 +304,7 @@ static const LG_InputOps inputOps = {
   .keyDown       = inputKey,
   .keyUp         = inputKey,
   .mouseMotion   = inputMouseMotion,
+  .mousePosition = inputMousePosition,
   .mousePress    = inputMouseButton,
   .mouseRelease  = inputMouseButton,
 };
@@ -482,6 +493,24 @@ static void testExitGuest(void)
   CHECK(count(EV_GUEST) == 0);
 }
 
+static void testAbsoluteSync(void)
+{
+  reset();
+  setLocal(50, 50);
+
+  core_handleGuestMouseUpdate();
+  CHECK(count(EV_GUEST) == 1);
+
+  bool absolute = true;
+  lgInput_setFallback(&inputOps, &absolute);
+  m.count = 0;
+
+  core_handleGuestMouseUpdate();
+  CHECK(count(EV_GUEST) == 0);
+
+  lgInput_setFallback(&inputOps, NULL);
+}
+
 static void testExitReentry(void)
 {
   startExit();
@@ -895,6 +924,7 @@ static const struct Test tests[] = {
   { "surface-exit"    , testSurfaceExit  },
   { "exit-immediate"  , testExitImmediate},
   { "exit-guest"      , testExitGuest    },
+  { "absolute-sync"   , testAbsoluteSync },
   { "exit-reentry"    , testExitReentry  },
   { "view-immediate"  , testViewImmediate},
   { "capture-pending" , testCapWait      },