diff --git a/src/ps2.js b/src/ps2.js
--- a/src/ps2.js
+++ b/src/ps2.js
@@ -34,6 +34,11 @@ export function PS2(cpu, bus)
         this.mouse_send_delta(data[0], data[1]);
     }, this);

+    this.bus.register("mouse-flush", function()
+    {
+        this.flush_mouse_movement(true);
+    }, this);
+
     this.bus.register("mouse-wheel", function(data)
     {
         if(!this.have_mouse || !this.use_mouse || !this.mouse_stream_active())
@@ -298,6 +303,41 @@ PS2.prototype.kbd_send_code = function(code)
     }
 };

+PS2.prototype.flush_mouse_movement = function(force)
+{
+    if(!this.mouse_stream_active())
+    {
+        return false;
+    }
+
+    var change_x = this.mouse_delta_x | 0,
+        change_y = this.mouse_delta_y | 0;
+
+    if(!change_x && !change_y ||
+        !force && (this.mouse_buffer.length ||
+            this.last_mouse_packet >= 0 &&
+            Date.now() - this.last_mouse_packet < 1000 / this.sample_rate))
+    {
+        return false;
+    }
+
+    do
+    {
+        // Keep every emitted packet representable as signed PS/2 deltas. A
+        // forced button boundary drains all chunks before changing buttons.
+        change_x = Math.max(-255, Math.min(255, change_x));
+        change_y = Math.max(-255, Math.min(255, change_y));
+        this.mouse_delta_x -= change_x;
+        this.mouse_delta_y -= change_y;
+        this.send_mouse_packet(change_x, change_y);
+        change_x = this.mouse_delta_x | 0;
+        change_y = this.mouse_delta_y | 0;
+    }
+    while(force && (change_x || change_y));
+
+    return true;
+};
+
 PS2.prototype.mouse_send_delta = function(delta_x, delta_y)
 {
     if(!this.have_mouse || !this.use_mouse)
@@ -313,26 +353,7 @@ PS2.prototype.mouse_send_delta = function(delta_x, delta_y)
     this.mouse_delta_x += delta_x * factor;
     this.mouse_delta_y += delta_y * factor;

-    if(this.mouse_stream_active())
-    {
-        var change_x = this.mouse_delta_x | 0,
-            change_y = this.mouse_delta_y | 0;
-
-        if(change_x || change_y)
-        {
-            //var now = Date.now();
-            //if(now - this.last_mouse_packet < 1000 / this.sample_rate)
-            //{
-            //    // TODO: set timeout
-            //    return;
-            //}
-
-            this.mouse_delta_x -= change_x;
-            this.mouse_delta_y -= change_y;
-
-            this.send_mouse_packet(change_x, change_y);
-        }
-    }
+    this.flush_mouse_movement(false);
 };

 PS2.prototype.mouse_send_click = function(left, middle, right)
@@ -342,12 +363,16 @@ PS2.prototype.mouse_send_click = function(left, middle, right)
         return;
     }

-    this.mouse_clicks = left | right << 1 | middle << 2;
+    var clicks = left | right << 1 | middle << 2;

     if(this.mouse_stream_active())
     {
+        // Motion received before this boundary belongs to the old buttons.
+        this.flush_mouse_movement(true);
+        this.mouse_clicks = clicks;
         this.send_mouse_packet(0, 0);
     }
+    this.mouse_clicks = clicks;
 };

 PS2.prototype.send_mouse_packet = function(dx, dy)
@@ -435,8 +460,13 @@ PS2.prototype.port60_read = function()
     if(this.next_byte_is_aux)
     {
         this.cpu.device_lower_irq(12);
         this.last_port60_byte = this.mouse_buffer.shift();
+        if(!this.mouse_buffer.length)
+        {
+            // Queue the next sampled movement only after the guest catches up.
+            this.flush_mouse_movement(false);
+        }
         dbg_log("Port 60 read (mouse): " + h(this.last_port60_byte), LOG_PS2);
     }
     else
     {