I noticed that a simple collapsed dock I have is still showing a bit of the contents when the page loads. If I expand it and collapse it, then it will display properly. Version I have is 2019.2.514
I checked and noticed it was using the Lightweight render mode and that it would be missing "overflow:hidden" in the css in the initial page load.
Code
<telerik:RadDockLayout runat=
"server"
ID=
"DockLayoutExceptions"
>
<telerik:RadDockZone runat=
"server"
ID=
"DockZoneException"
Orientation=
"Vertical"
MinHeight=
"40px"
MinWidth=
"300px"
>
<telerik:RadDock RenderMode=
"Lightweight"
runat=
"server"
ID=
"DockExceptions"
Title=
"Exceptions"
EnableDrag=
"False"
DockMode=
"Docked"
Collapsed=
"True"
>
<Commands>
<telerik:DockExpandCollapseCommand />
</Commands>
<ContentTemplate>
<cust:ExceptionTable runat=
"server"
ID=
"tblOrderExceptions"
Type=
"Order"
/>
</ContentTemplate>
</telerik:RadDock>
</telerik:RadDockZone>
</telerik:RadDockLayout>
Attached are images showing the generated CSS at the page load and after expanding and collapsing again.