I've WebCombo V 6, and I setup the width to 100%. Onlyi if I setup the width to any certain percentage, the arrow disappears after first time the user click on the arrow to open the drop down list.

Could you confirm whether you are using the latest assembly of WebCombo 6 and WebUI.NET Framework 3.0? I tried to reproduce the problem in my local tests by editing the DefaultStyle.aspx sample file of WebCombo (set the width of WebCombo to a certain percentage, 80%) but my efforts were not successful -- everything worked smoothly and the arrow was rendered without any issues.

For your information, I'm using WebCombo 6 build 260 and WebUI.NET Frameowkr 3 build 913. The sample page is tested on IE 11 browser.

I found a work around, but I still believe you need to fix this behavior.
I call OnShowDropDown(), and I call ISGetObject(comboID).DoResize();

Invoke DoResize() method can be used as work around for this kind of issue. Please kindly assist me to be able to reproduce the problem to be investigated further the development team.