DragPanelExtender¶

The DragPanelExtender makes a panel or container element draggable. Users can click and drag the panel to reposition it freely on the page. Optionally, a specific handle or title bar can be designated as the drag grip, restricting dragging to that element.

Original Ajax Control Toolkit documentation: https://www.asp.net/ajax/ajaxcontroltoolkit/DragPanelExtender

Features Supported in Blazor¶

TargetControlID — ID of the panel or container to make draggable
DragHandleID — Optional ID of the element to use as the drag handle
Enabled — Enable or disable the extender behavior
BehaviorID — Optional identifier for JavaScript behavior lookup

Web Forms Syntax¶

<asp:Panel ID="pnlDialog" runat="server" style="border: 1px solid black; width: 300px;">
    <div id="pnlHeader" style="background-color: #333; padding: 5px;">
        <h3>My Dialog</h3>
    </div>
    <div style="padding: 10px;">
        <p>Drag me by the header!</p>
    </div>
</asp:Panel>

<ajaxToolkit:DragPanelExtender
    ID="drag1"
    runat="server"
    TargetControlID="pnlDialog"
    DragHandleID="pnlHeader" />

Blazor Migration¶

<Panel ID="pnlDialog" style="border: 1px solid black; width: 300px;">
    <div id="pnlHeader" style="background-color: #333; padding: 5px;">
        <h3>My Dialog</h3>
    </div>
    <div style="padding: 10px;">
        <p>Drag me by the header!</p>
    </div>
</Panel>

<DragPanelExtender
    TargetControlID="pnlDialog"
    DragHandleID="pnlHeader" />

Migration is simple: Remove the ajaxToolkit: prefix and runat="server". Everything else stays the same!

Properties Reference¶

Property	Type	Default	Description
`TargetControlID`	`string`	(required)	ID of the panel or container to make draggable
`DragHandleID`	`string`	`""`	Optional ID of an element to use as the drag handle; if not set, entire panel is draggable
`BehaviorID`	`string`	TargetControlID	Optional identifier for JavaScript behavior lookup
`Enabled`	`bool`	`true`	Whether the extender is active

Usage Examples¶

Draggable Entire Panel¶

@rendermode InteractiveServer

<Panel ID="pnlBox" style="width: 300px; border: 1px solid #ccc; padding: 10px; background: white; cursor: move;">
    <p>Drag this panel anywhere!</p>
</Panel>

<DragPanelExtender TargetControlID="pnlBox" />

Draggable with Handle¶

@rendermode InteractiveServer

<Panel ID="pnlWindow" style="width: 400px; border: 1px solid black;">
    <div id="titleBar" style="background-color: #4CAF50; color: white; padding: 10px; cursor: move;">
        Window Title
    </div>
    <div style="padding: 20px; background-color: white;">
        <p>This can only be dragged by the title bar.</p>
    </div>
</Panel>

<DragPanelExtender
    TargetControlID="pnlWindow"
    DragHandleID="titleBar" />

Floating Dialog Box¶

@rendermode InteractiveServer

<Panel ID="pnlDialog" style="position: absolute; top: 100px; left: 100px; width: 350px; border: 1px solid #999; box-shadow: 0 2px 5px rgba(0,0,0,0.2); background: white;">
    <div id="dialogHeader" style="background: linear-gradient(to right, #2196F3, #1976D2); color: white; padding: 10px; font-weight: bold; cursor: move;">
        Settings
    </div>
    <div style="padding: 20px;">
        <form>
            <input type="checkbox" /> Enable notifications
            <br /><br />
            <button type="button" class="btn btn-primary">Save</button>
        </form>
    </div>
</Panel>

<DragPanelExtender
    TargetControlID="pnlDialog"
    DragHandleID="dialogHeader" />

HTML Output¶

The DragPanelExtender produces no HTML itself — it attaches JavaScript behavior to the target panel. The panel retains its original HTML structure and styling.

JavaScript Interop¶

The DragPanelExtender loads drag-panel-extender.js as an ES module. JavaScript handles:

Detecting mouse down on the panel or handle
Tracking mouse movement while dragging
Updating the panel's position in real-time
Handling mouse up to stop dragging
Managing z-index to keep dragged panel on top
Preventing text selection during drag

Render Mode Requirements¶

The DragPanelExtender requires InteractiveServer render mode:

@rendermode InteractiveServer

Graceful Degradation¶

SSR/Static mode: The extender silently skips initialization. The Panel displays statically.
JavaScript disabled: Same as SSR — Panel is not draggable.

Migration Notes¶

From Web Forms Ajax Toolkit¶

Remove ajaxToolkit: prefix

- <ajaxToolkit:DragPanelExtender
+ <DragPanelExtender

Remove runat="server" and ID attributes

Panel element stays the same

- <asp:Panel ID="pnlDialog" runat="server">
+ <Panel ID="pnlDialog">

Before (Web Forms)¶

<asp:Panel ID="pnlDialog" runat="server" style="width: 300px; border: 1px solid black;">
    <div id="header" style="background: #333; color: white; padding: 5px; cursor: move;">Title</div>
    <div style="padding: 10px;">Content</div>
</asp:Panel>

<ajaxToolkit:DragPanelExtender
    ID="drag1"
    TargetControlID="pnlDialog"
    DragHandleID="header"
    runat="server" />

After (Blazor)¶

<Panel ID="pnlDialog" style="width: 300px; border: 1px solid black;">
    <div id="header" style="background: #333; color: white; padding: 5px; cursor: move;">Title</div>
    <div style="padding: 10px;">Content</div>
</Panel>

<DragPanelExtender
    TargetControlID="pnlDialog"
    DragHandleID="header" />

Best Practices¶

Set position: absolute — Use CSS to position the panel before dragging starts
Use a handle — Provide a DragHandleID to prevent accidental dragging of content
Apply cursor: move — Use CSS to indicate the element is draggable
Preserve boundaries — Monitor panel position to keep it within viewport
Apply z-index — Ensure dragged panels appear above other page content

Troubleshooting¶

Issue	Solution
Panel not draggable	Verify `TargetControlID` matches the Panel's `ID`. Ensure `@rendermode InteractiveServer` is set.
Dragging causes text selection	Add `user-select: none` CSS or use `DragHandleID` to restrict dragging to a specific element.
Handle not working as expected	Ensure `DragHandleID` references a valid element within the panel with the correct ID.
Panel disappears during drag	Check z-index values and ensure the panel's position is compatible with absolute positioning.