diff --git a/Apps/Explorer/desk_server.vp b/Apps/Explorer/desk_server.vp
index 5cfcfe1..eb0135a 100644
--- a/Apps/Explorer/desk_server.vp
+++ b/Apps/Explorer/desk_server.vp
@@ -11,6 +11,8 @@ import viperlib
 
 # 【诊断·临时】首个鼠标包已收到的标记（0 = 未收到）
 _mouse_logged: int = 0
+# 首次 vblank 等待的迭代数只打一次（诊断同步是否真的生效；到上限 = 超时）
+_vblank_logged: int = 0
 
 
 # ============================================================================
@@ -661,17 +663,26 @@ class DeskServer:
 
     def render(self):
         if self.dirty_any == 0: return
-        # 写屏前等垂直回扫（用户态不能 inb 0x3DA，由内核代等），避免撕裂
-        devfs.fb_wait_vblank(self.fb_fd)
+        y0: int = -1
+        y1: int = -1
         y: int = 0
         while y < self.fb_h:
-            if self.dirty[y] == 0:
-                y += 1
-                continue
-            y0: int = y
-            while y < self.fb_h and self.dirty[y] != 0:
-                y += 1
-            self.render_range(y0, y)
+            if self.dirty[y] != 0:
+                if y0 < 0: y0 = y
+                y1 = y + 1
+            y += 1
+        if y0 < 0:
+            self.clear_dirty()
+            return
+        # 写屏前等垂直回扫（用户态无 IO 权限，由内核代等）；脏行合并成**一段**连续范围
+        # 一次性渲染 —— 分段渲染会让同一帧的多次写屏跨越多个扫描周期，撕裂更明显。
+        vw: int = devfs.fb_wait_vblank(self.fb_fd)
+        if _vblank_logged == 0:
+            _vblank_logged = 1
+            _vb: [96]char
+            viperlib.snprintf(&_vb, 96, "[explorer] vblank iters=%d\n", vw)
+            devfs.serial_puts(&_vb)
+        self.render_range(y0, y1)
         self.clear_dirty()
 
     # ---------------- 事件投递 ----------------
diff --git a/VKernel/Kernel/drivers/devfs/devfs.vp b/VKernel/Kernel/drivers/devfs/devfs.vp
index 1daa36f..0a41bc8 100644
--- a/VKernel/Kernel/drivers/devfs/devfs.vp
+++ b/VKernel/Kernel/drivers/devfs/devfs.vp
@@ -522,12 +522,13 @@ virtual class framebuffer_device(devfs_file):
 
     def ioctl(self, fd: int, request: int, arg: rawptr) -> int:
         if request == FB_WAIT_VBLANK:
-            # 等进入垂直回扫期（0x3DA bit3）。带超时：当前若无 VGA 时序不至于死等。
+            # 等进入垂直回扫期（0x3DA bit3）。返回等待消耗的迭代数：到上限即超时
+            #（当前没有 VGA 回扫信号），调用方可据此判断同步是否真的生效。
             t: int = 100000
             while (asm.inb(0x3DA) & 0x08) != 0 and t > 0: t -= 1
-            t = 100000
-            while (asm.inb(0x3DA) & 0x08) == 0 and t > 0: t -= 1
-            return 0
+            t2: int = 100000
+            while (asm.inb(0x3DA) & 0x08) == 0 and t2 > 0: t2 -= 1
+            return 100000 - t2
         if request != FB_GET_INFO or not arg:
             return -1
         info: FbInfo* = FbInfo*(uint64(arg))