There is at least one IE8 compatibility mode that is undetectable on the server. Press F12 and check if your Browser Mode is IE8 Compat View and Document Mode is IE8 Standards - this particular mode selection should be impossible in the wild unless Developer Tools is used to select it.
However one can manage to trigger it if local intranet is forced to open by default in compatibility view (clean IE8 after installation has this option enabled) and has forced IE8 browser mode with one of these meta tags in your pages:
<meta http-equiv="X-UA-Compatible" content="IE=Edge" />
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8" />
<meta http-equiv="X-UA-Compatible" content="IE=8" />
This combination will always trigger this compatibility mode bug while browsing in local intranet.
There are two solutions to this issue - the first is obviously to stop the compatibility mode for local intranet in the Compatibility View settings but this should be done on every computer which should access the site. The other is to sacrifice the horizontal scrolling of RadScheduler by adding the following (very hacky) CSS to your stylesheet:
width /*\**/: 100%\9 !important;
*+html div.RadScheduler .rsHorizontalHeaderTable,
*+html div.RadScheduler .rsContentTable
width: auto !important;
It should work in all browsers, including all IE8 and all IE9 compatibility modes which I tested. Let me know if this helps.
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