The ContentScrolling property does not take effect under IE10. The end result is that the control behaves as if the property has its Default value (i.e. the browser will handle the overflow of the elements, which means large content will stretch the tooltips).

When the following properties are set - HideEvent="LeaveToolTip" and RenderMode="LightWeight" and the mouse cursor is moved over the ToolTip in a slower motion, the tooltip itself is hidden.

Problem:

With the latest version of Google Chrome 61 the tooltip is not positioning correctly when the page is scrolled down.

Details: 

The tooltip positioning problem is due to the following breaking change in Chrome 61 (see release notes at https://blog.chromium.org/2017/08/chrome-61-beta-javascript-modules.html):

To align with the spec and preserve browser consistency, the scrollingElement is now the documentElement in standards mode.
Chrome 61 has changed the behavior of document.scrollingElement to return document.documentElement instead of document.body to match the CSSOM View specification and this broke the positioning of the tooltip.

Solution:

Use the _getPosRelativeToMouse override to solve the problem:

<telerik:RadGrid ID="RadGrid" runat="server"></telerik:RadGrid>
<telerik:RadToolTipManager ID="RadToolTipManager1" AutoTooltipify="true" runat="server">
</telerik:RadToolTipManager>
   Telerik.Web.UI.RadToolTip.prototype._getPosRelativeToMouse = function(targetBounds) {
                 var elemX = targetBounds.x;
                 var elemY = targetBounds.y;

                 //Get last recorded mouse position
                 var pos = this._getMousePosition();
                 var mouseX = pos.clientX;
                 var mouseY = pos.clientY;

                 //Take into consideration the offsetScroll!
                 var standard = $telerik.standardsMode;
                 //$telerik.standardsMode does not do a good job! Extra check is needed for FF!!
                 //And yet another check needed for Safari! It should always be set to false in order to get the calculations right
                 if (!$telerik.isIE && document.compatMode != "CSS1Compat") standard = false;
                 else if (($telerik.isSafari && !(Telerik.Web.Browser.chrome && Telerik.Web.Browser.version >= 61)) || (Telerik.Web.Browser.edge && Telerik.Web.Browser.version >= 15)) standard = false;

                 if (standard) {
                     elemX -= $telerik.getCorrectScrollLeft(document.documentElement);
                     elemY -= document.documentElement.scrollTop;
                 }
                 else //NEW: Add support for quircksmode
                 {
                     elemX -= $telerik.getCorrectScrollLeft(document.body);
                     elemY -= document.body.scrollTop;
                 }

                 //Calculate the position of the mouse, relative to the targetcontrol
                 var deltaX = mouseX - elemX;
                 var deltaY = mouseY - elemY;

                 return { x: deltaX, y: deltaY };
             }

When Tooltip ContentScrolling property is set to Auto the the tooltip dimensions are not computed properly and the layout looks inconsistent (i.e. usually the left and/or right borders are missing).

Possible workarounds are:

1) using content that will fit in the tooltip and leaving the ContentScrolling property to its default value

OR

2) using some JavaScript to modify the dimensions:
function OnClientResponseEnd()
{
	var activeTooltip = Telerik.Web.UI.RadToolTip.getCurrent();
	if (activeTooltip)
	{
		var contentElement = activeTooltip.get_contentElement();
		if (contentElement)
		{
			contentElement.style.width = parseInt(contentElement.style.width) - 15 + "px";
			contentElement.style.height = parseInt(contentElement.style.height) - 15 + "px";
			contentElement.style.overflow = "auto";
		}
	}
}

where this is attached to the OnClientResponseEnd event of the RadToolTipManager.

See http://www.telerik.com/community/forums/aspnet-ajax/tooltip/changing-targetcontrol-on-the-client.aspx
It should be expected that this timeout around the public API is not needed and the problem is taken care of internally by the control.

A workaround is to force the tooltip to update its position when the content arrives (i.e., in the OnClientResponseEnd event: http://www.telerik.com/help/aspnet-ajax/tooltipmanager-client-side-on-response-end.html )

function OnClientResponseEnd(sender, args) {
    var activeTooltip = Telerik.Web.UI.RadToolTip.getCurrent();
    if (activeTooltip) {
        activeTooltip.updateLocation();
    }
}

When the page is in RTL mode and the manual close button of the RadToolTip is hovered it changes its position slightly.

Telerik version - 2011.1.614.35

RadToolTip issue seen on Chrome (version 28.0.1500.95) and Safari (version 5.1.7)

The RadToolTip doesn’t align properly with the control that was hidden initially.
i.  There is a label control inside the RadGrid header template, that has RadToolTip attached.
ii.  On page load, the column is hidden.
iii.  The column is made visible when a particular button is clicked (client side event).
iv.  When the cursor is hovered over this column for the first time, the tool tip is seen at a distant place away from the label.
v.  However, when hovered again, it is seen properly aligned with the label control

A possible workaround is to add the background-color via CSS:
		div.RadToolTip.RadToolTip_Office2007,
		.RadToolTip_Office2007 div.rtContent
		{
			background-color: #d7e3f2;
		}

In case the color difference is a problem, you can examine the rendered Classic tooltip and find the color CSS rule that is applied, then apply it to the Lightweight mode via a CSS selector similar to the following:
		div.RadToolTip_Simple .rtContent
		{
			color: #C98400;
		}
Where you can change Simple with the skin name that you are using.

The workaround is to avoid the other settings (e.g., Auto). In most cases Lightweight RenderMode will suffice, the only potential issue is loss of gradients, shadows and rounded corners in IE6-8.

The issue stems from the content div element that has explicit background color. The loading sign is a background-image on the main tooltip wrapper, so having explicit height and/or scrolling causes the later element to hide the root element, thus, hiding the loading sign as well.

You can work around this by removing the explicit color declaration for all skins:

div.RadToolTip div.rtContent
{
	background-color: transparent;
}

When the tooltip is behind the modality it cannot be closed by the users. 

You can workaround this behavior by following the script below:

<telerik:RadButton runat="server" ID="RadButton1" Text="Hover"></telerik:RadButton>

<telerik:RadToolTip ID="rttAdditionalFeatures" runat="server" ManualClose="True" Modal="True"
    TargetControlID="RadButton1" ShowEvent="OnMouseOver" OnClientShow="OnClientShow">
    Some text
</telerik:RadToolTip>

<script type="text/javascript">
    function OnClientShow(sender, args) {
        var modalLayerZindex = sender._modalExtender._backgroundElement.style.zIndex;

        if (sender.get_zIndex() <= modalLayerZindex) {
            sender.get_popupElement().style.zIndex = modalLayerZindex + 1;
        }
    }
</script> 

The RadTooltip and RadTooltipManager do not display tooltips like the default browser tooltips, if the tooltips are "nested".

Consider HTML like this

My name is <span class="app" title="Person Tooltip">Mathyß<a class="noteLink criticalRight" id="RqoTeiLink_a" title="Der Notelink Tooltip"><sup>a</sup></a> Schwertzenbachen</span>, and I live somewhere. 

If you tooltipify this HTML then only the "outer" tooltip is displayed all the time. When using just the browser, then it displays correctly the "inner" tooltip when you hover over the link.

As the behaviour is not consitent with the browser (and other Tooltip libraries) this should be fixed.

A possible workaround is to recalculate the positions in the OnClientShow event. Here is an example for the BottomCenter position

					<script>
						function repositionCallout(sender, args) {
							var callout = $telerik.$(".rtCallout.rtCalloutTopCenter", sender.get_popupElement());
							if (callout.length > 0) { //there is a callout and its position is in the top center (i.e., tooltip's position is BottomCenter)
								var targetPos = $telerik.getBounds(sender.get_targetControl());
								var ttipPos = $telerik.getBounds(sender.get_popupElement());
								var calloutPos = $telerik.getBounds(callout[0]);
								var desiredCalloutLeft = targetPos.x - ttipPos.x + targetPos.width / 2 - calloutPos.width / 2;
								callout.css("left", desiredCalloutLeft);
							}
						}
					</script>
					<telerik:RadToolTip ID="MainMenu" runat="server" TargetControlID="imgbtnMenuIcon" ShowEvent="OnClick" ShowDelay="0" AutoCloseDelay="0" Position="BottomCenter" RelativeTo="Element" RenderMode="Lightweight" Width="500px" Height="500px" OnClientShow="repositionCallout">
						tooltip content
					</telerik:RadToolTip>

When a large page is scrolled to the target element, the tooltip is placed higher than expected.

Possible workaround is to use RelativeTo="Element" property.