I do not have the same issue as you, but there do seem to be some strange layout glitches in versions 3 and 4. For example, if you have a form containing a tab container that is wider than the default pop-up width it draws like this with right-left scroll controls:
These scroll controls are not actually needed as the dialog is set to render at a larger width (800 wide instead of the default 600). If you click the restore control the tab is then correctly displayed like this:
It seems the calculation on whether to include the scroll controls is done using the default layout size, rather than the final dialog size specified. I wonder whether something similar is causing your issue, in other words, is something causing a resizing of your layout after the scrollbar has been positioned?
To be more precise the troubled code is this:
<div id="mxui_wm_ContentManager_0" widgetid="mxui_wm_ContentManager_0" style="overflow: auto;" class="MxClient_contentManager" focuscycle="true">
After a resize the style attribute disappears and the scrollbars are as expected.
Samet, maybe you can add this in your original post?
We also have a similar issue with tabcontainers. Opening a tabcontainer within a tabcontainer, shows three bars. Resizing the window (even only by 1px) solves the issue temporarily.