perf(explorer): per-frame window double buffering and present-aware FLUSH

Apps/Explorer/desk_server.vp

@@ -854,6 +854,9 @@ class DeskServer:
                 cap_px: int = w * content_h
                 buf_off: uint32 = desk.desk_arena_alloc(self.shm, uint32(cap_px) * uint32(4))
                 if buf_off == uint32(0): return -1
+                # 第二块内容缓冲：应用始终画在非显示缓冲，FLUSH 后交换。
+                buf2_off: uint32 = desk.desk_arena_alloc(self.shm, uint32(cap_px) * uint32(4))
+                if buf2_off == uint32(0): buf2_off = buf_off
                 s.used = 1
                 s.pid = pid
                 s.active = 1
@@ -873,18 +876,22 @@ class DeskServer:
                 s.fb_cap = uint32(cap_px)
                 s.fb_w = uint32(w)
                 s.fb_h = uint32(content_h)
-                self.vis_off[i] = buf_off
+                self.vis_off[i] = buf2_off
                 self.vis_cap[i] = uint32(cap_px)
                 self.vis_w[i] = w
                 self.vis_h[i] = content_h
                 self.spare_off[i] = uint32(0)
                 self.spare_cap[i] = uint32(0)
                 string.memset(&s.title, 0, desk.DESK_TITLE_MAX)
-                # 像素缓冲先清成窗口底色，避免 app 还没画就露出垃圾
+                # 两块缓冲都清成窗口底色，避免 app 还没画就露出垃圾
                 buf: uint32* = uint32*(self.shm + uint64(buf_off))
                 col: uint32 = self.rgb(uint32(30), uint32(30), uint32(50))
                 for k in range(cap_px):
                     buf[k] = col
+                if buf2_off != buf_off:
+                    buf2: uint32* = uint32*(self.shm + uint64(buf2_off))
+                    for k2 in range(cap_px):
+                        buf2[k2] = col
                 s.z_order = self.next_z
                 self.next_z += 1
                 self.focused = i
@@ -1998,16 +2005,35 @@ class DeskServer:
                 if self.hit_test(self.cx, self.cy) == win:
                     self._mark_cursor_band()
         elif ty == uint32(desk.DESK_REQ_FLUSH):
-            # 应用已经按 s.fb_w/fb_h 画完一帧：此刻才把"正在显示的内容"切到
-            # 新缓冲。旧的 vis 缓冲留作下一次 resize 的备用。
+            # present=0：PING/保活，不交换缓冲，避免显示未绘制的新缓冲。
+            if m.a == 0:
+                return
             old_off: uint32 = self.vis_off[win]
-            if old_off != s.fb_off:
-                self.spare_off[win] = old_off
-                self.spare_cap[win] = self.vis_cap[win]
-            self.vis_off[win] = s.fb_off
-            self.vis_cap[win] = s.fb_cap
-            self.vis_w[win] = int(s.fb_w)
-            self.vis_h[win] = int(s.fb_h)
+            old_cap: uint32 = self.vis_cap[win]
+            need: int = int(s.fb_w) * int(s.fb_h)
+            if s.fb_off != old_off:
+                # 本帧画在新的 offscreen 缓冲里：切到它，旧 vis 作为下一帧目标。
+                self.vis_off[win] = s.fb_off
+                self.vis_cap[win] = s.fb_cap
+                self.vis_w[win] = int(s.fb_w)
+                self.vis_h[win] = int(s.fb_h)
+                if old_off >= uint32(desk.DESK_OFF_ARENA) and int(old_cap) >= need:
+                    s.fb_off = old_off
+                    s.fb_cap = old_cap
+                else:
+                    nw: uint32 = desk.desk_arena_alloc(self.shm, uint32(need * 4))
+                    if nw != uint32(0):
+                        s.fb_off = nw
+                        s.fb_cap = uint32(need)
+                    else:
+                        s.fb_off = old_off
+                        s.fb_cap = old_cap
+            else:
+                # 首帧或单缓冲回退：vis 已经是当前内容，为下一帧准备第二块。
+                nw2: uint32 = desk.desk_arena_alloc(self.shm, uint32(need * 4))
+                if nw2 != uint32(0):
+                    s.fb_off = nw2
+                    s.fb_cap = uint32(need)
             # 最小化窗口仍在后台绘制/FLUSH：不要置脏，否则合成器每帧都要重画背景。
             if s.minimized == 0:
                 self.mark(s.uy, s.h)

vpsdk/vpui.vp

@@ -2050,7 +2050,7 @@ virtual class Tk(Wm, Misc):
                     need_redraw: int = 0
                     etype: uint32 = evt.type
                     if etype == window.EVENT_TYPE_PING:
-                        window.flush(self.win_id)
+                        window.flush(self.win_id, 0)
                     elif etype == window.EVENT_TYPE_RESIZE:
                         self.buf = window.get_winbuf(self.win_id)
                         sz: int = window.get_win_size(self.win_id)

vpsdk/window.vp

@@ -183,13 +183,13 @@ def get_win_size(win_id: int) -> int:
     if s.used == 0: return 0
     return (int(s.fb_w) << 16) | (int(s.fb_h) & 0xFFFF)
 
-def flush(win_id: int):
+def flush(win_id: int, present: int = 1):
     # 槽位已被 explorer 回收（用户点了关闭）：直接安全返回，绝不再发合成请求。
     if _connect() != 0: return
     if win_id < 0 or win_id >= desk.DESK_MAX_WIN: return
     s: desk.WinSlot* = _slot(win_id)
     if s.used == 0: return
-    _send(uint32(desk.DESK_REQ_FLUSH), win_id, 0, 0, 0, 0, None)
+    _send(uint32(desk.DESK_REQ_FLUSH), win_id, present, 0, 0, 0, None)
 
 def text(win_id: int, x: int, y: int, color: uint32, s: const str) -> int:
     if _connect() != 0: return -1