I have an obout ColorPickerExtender in an UpdatePanel along with some other controls. The other controls execute partial postbacks as expected, but the ColorPickerExtender executes a full postback despite being in the UpdatePanel. Here's the relevant ASPX:
<asp:Content ContentPlaceHolderID="cphMainDivContentPlaceHolder" runat="server">
<asp:UpdatePanel ID="upGeneralLayoutData" runat="server">
<ContentTemplate>
<asp:TextBox ID="txtLayoutName" runat="server"
ToolTip="Enter a name for this layout (recommend you use a unique name)"
OnTextChanged="txtLayoutName_TextChanged"
AutoPostBack="true"
MaxLength="255" />
<obout:ColorPickerExtender ID="cpeLayoutBackgroundColor" runat="server"
OnClientOpen="onColorPickerExtenderOpen"
AutoPostBack="true"
TargetProperty="style.backgroundColor"
OnColorPostBack="cpeLayoutBackgroundColor_ColorPostBack"
PopupButtonID="txtLayoutBackgroundColor"
TargetControlID="txtLayoutBackgroundColor"
HexView="False"
PickButton="False" />
<asp:TextBox ID="txtLayoutBackgroundColor" runat="server"
ToolTip="Select the background color for this layout"
CssClass="ColorPickerExtenderTextBox"
style="cursor: pointer"
Width="50"
ReadOnly="True" />
<br />
</ContentTemplate>
</asp:UpdatePanel>
</asp:Content>
As I was formulating the question, I was able to figure out the answer (see below)--instead of trashing the question, I left it here for others to use.
It turns out that the ColorPickerExtender is not being registered as an async postback control. I got the clue from this post. I'm not sure why it doesn't register as an async control when the others do, but the fix is easy enough--add a <Triggers> section that explicitly designates it as async, like so:
<Triggers>
<asp:AsyncPostBackTrigger ControlID="cpeLayoutBackgroundColor" EventName="ColorPostBack" />
</Triggers>