[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/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;
-  (void)support;
-  return false;
+  return support == LG_INPUT_SUPPORT_MOUSE_ABSOLUTE &&
+    opaque && *(const bool *)opaque;
 }
 
 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;
@@ -289,13 +299,14 @@ static bool inputMouseButton(void * opaque, unsigned int button)
 }
 
 static const LG_InputOps inputOps = {
-  .name         = "test",
-  .supports     = inputSupports,
-  .keyDown      = inputKey,
-  .keyUp        = inputKey,
-  .mouseMotion  = inputMouseMotion,
-  .mousePress   = inputMouseButton,
-  .mouseRelease = inputMouseButton,
+  .name          = "test",
+  .supports      = inputSupports,
+  .keyDown       = inputKey,
+  .keyUp         = inputKey,
+  .mouseMotion   = inputMouseMotion,
+  .mousePosition = inputMousePosition,
+  .mousePress    = inputMouseButton,
+  .mouseRelease  = inputMouseButton,
 };
 
 static void reset(void)
@@ -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      },