Reproduce:

Use the TestComposer.java, index.zul and test.xlsx in testing_sample.zip attached.
Use the arguments below to limit memory to 512MB.

-Xms512m -Xmx512m -XX:NewSize=128m -XX:MaxNewSize=128m -XX:PermSize=512m -XX:MaxPermSize=512m

Open about 10 pages of index.zul in 3 different browsers
Select whole sheet in each page
Use different text color in each page
Click 'text color' button quickly, repeatedly, in each page.

Sometimes it throws ConcurrentModificationException, ignore it here.
After several runs the JVM will starts GC forever, and shows OutOfMemory.

This is considered as an issue since only 10 pages of small sheet (30 x 70), and reproducible with one person (just need to perform operations quickly).

The content of OOM Exceptions can be vary since it can be thrown everywhere.

For more information please refer to visualvm.png and stacktrace.txt attached.

Workaround:
Decrease concurrent thread,

e.g.
Decrease "maxThreads" and increase "acceptCount" of the Connector defined in tomcat-folder/conf/server.xml

<Connector connectionTimeout="20000" port="8080"
    protocol="HTTP/1.1" redirectPort="8443"
    maxThreads="20" acceptCount="2000" />