Position the <img> cursor relative to the client plugin, not the window.

remoting/webapp/base/html/main.css

Index: remoting/webapp/base/html/main.css
diff --git a/remoting/webapp/base/html/main.css b/remoting/webapp/base/html/main.css
index 19f426f7dda49c6732b883705dfbd286bfb203c8..6dfd1b622c57ccaba4fa23403ce6d6a113eb4030 100644
--- a/remoting/webapp/base/html/main.css
+++ b/remoting/webapp/base/html/main.css
@@ -733,6 +733,12 @@ body:not(.connected) #session-mode {
   box-sizing: border-box;
 }
 
+#client-container {
+  /* The mouse-cursor-overlay is positioned relative to the client plugin,
+   * which is contained within this element. */
+  position: relative;
+}
+
 .mouse-cursor-overlay {
   position: absolute;
   pointer-events: none;