<TelerikWindow Width="80%"
                Height="auto"
                Top="404px"
                State="WindowState.Minimized"
                Left="200px"                        
                Class="myClass"
                Visible="true"
><WindowTitle>My Title</WindowTitle><WindowActions><WindowAction Name="Minimize"></WindowAction></WindowActions><WindowContent><h1>My Content</h1></WindowContent></TelerikWindow>
 
Window starts out minimized, with proper width, height, top and left.
Clicking maximize button clears the top and left property, and resets position every time you open/close.
Click/dragging to move the window will reapply top/left property, and then maximize/minimize works properly.
 
If you change the state to to WindowState.Maximized, it has the correct top/left property, but resets the width/height properties.
Window is not draggable.
 
Not setting the State at all is the only way to have proper behavior.
    
    
        
    
    
        
    

<TelerikWindow Visible="true" Class="workaround-for-touch-drag">
    <WindowTitle>the title</WindowTitle>
    <WindowContent>the content</WindowContent>
</TelerikWindow>

<style>
    .workaround-for-touch-drag {
        touch-action: none;
    }
</style>

<div style="height:400px;"></div>
<TelerikButton Icon="window" OnClick=@(()=> WindowVisible = true)>Open Modal</TelerikButton>
<div style="height:400px;"></div>

<TelerikWindow Class="demo-window" Width="500px" Height="300px" Centered="true" @bind-Visible=@WindowVisible Modal="true">
    <WindowTitle>
        <strong>Modal Window</strong>
    </WindowTitle>
    <WindowActions>
        <WindowAction Name="Close" />
    </WindowActions>
    <WindowContent>
        <p>Click the close button to dismiss this modal dialog.</p>
    </WindowContent>
</TelerikWindow>

@code { public bool WindowVisible { get; set; } }

<style type="text/css">
    .demo-window {
        z-index: 44444;
    }
</style>

<TelerikWindow Class="@( isMinimized ? "minimize-window" : "maximize-window" )"
               Width="500px"
               Height="@Height"
               Top="40%"
               Left="45%"
               Visible="true">
    <WindowTitle>
        <strong>Status</strong>
    </WindowTitle>
    <WindowActions>
        <WindowAction Name="MyCustomMinimize" OnClick="@CustomMinimize" Icon="@IconName.WindowMinimize"></WindowAction>
        @if (isMinimized)
        {
            <WindowAction Name="MyCustomMaximize" OnClick="@CustomMaximize" Icon="@IconName.Window"></WindowAction>
        }
        <WindowAction Name="Close"></WindowAction>
    </WindowActions>
    <WindowContent>
        <form class="k-form">
            <fieldset class="k-form-fieldset">
                <legend class="k-form-legend">User Details</legend>

                <label class="k-form-field">
                    <span>First Name</span>
                    <input class="k-textbox" placeholder="Your Name" />
                </label>
                <label class="k-form-field">
                    <span>Last Name</span>
                    <input class="k-textbox" placeholder="Your Last Name" />
                </label>
            </fieldset>
        </form>
    </WindowContent>
</TelerikWindow>

@code {
    public bool isMinimized { get; set; }

    public string Height { get; set; } = "350px";

    private void CustomMinimize()
    {
        isMinimized = true;
        Height = "auto";
    }

    private void CustomMaximize()
    {
        isMinimized = false;
        Height = "350px";
    }

}

<style>
    .minimize-window .k-content {
        display: none;
        padding: 0px !important;
    }

    .maximize-window .k-content {
        display: block;
        padding: 16px 16px;
    }
</style>

<TelerikWindow Left="@TheLeft" Top="@TheTop" Draggable="true"
               LeftChanged="@LeftChangedHandler" TopChanged="@TopChangedHandler"
               Visible="true" Width="400px" Height="300px">
    <WindowTitle>Drag me!</WindowTitle>
    <WindowContent>When using Left and Top, make sure to update them in the view-model.</WindowContent>
    <WindowActions>
        <WindowAction Name="Minimize"></WindowAction>
        <WindowAction Name="Maximize"></WindowAction>
    </WindowActions>
</TelerikWindow>

@code{
    string TheLeft { get; set; } = "50px";
    string TheTop { get; set; } = "50px";

    async Task LeftChangedHandler(string currLeft)
    {
        float leftValue = float.Parse(currLeft.Replace("px", ""));
        Console.WriteLine("left " + leftValue);
        //left boundary
        if (leftValue < 0)
        {
            currLeft = "0px";
        }
        //right boundary - replace hardcoded values with current values from the viewport size
        //and take into account the dimensions of your window. This sample hardcodes values for brevity and clarity
        //you may find useful packages like this to get the viewport size https://www.nuget.org/packages/BlazorPro.BlazorSize/
        if (leftValue > 1000)
        {
            currLeft = "1000px";
        }


        TheLeft = currLeft;

        await DelayedRender();
    }

    async Task TopChangedHandler(string currTop)
    {
        float topValue = float.Parse(currTop.Replace("px", ""));
        Console.WriteLine("top: " + topValue);

        //top boundary
        if (topValue < 0)
        {
            currTop = "0px";
        }
        //bottom boundary - replace hardcoded values with current values from the viewport size
        //and take into account the dimensions of your window
        if (topValue > 600)
        {
            currTop = "600px";
        }

        TheTop = currTop;

        await DelayedRender();
    }

    async Task DelayedRender()
    {
        await Task.Run(async () =>
        {
            await Task.Delay(30);
            await InvokeAsync(StateHasChanged);
        });
    }
}

<WindowAction Name="Maximize" Hidden="true"></WindowAction>

See this REPL for an example:
https://blazorrepl.telerik.com/wPbGYMvi23qjUo1c41

@result

<TelerikButton OnClick="@ToggleWindow">Toggle the Window</TelerikButton>

<TelerikWindow Visible="@isVisible" VisibleChanged="@VisibleChangedHandler">
    <WindowTitle>
        <strong>The Title</strong>
    </WindowTitle>
    <WindowContent>
        This is my window <strong>popup</strong> content.
    </WindowContent>
    <WindowActions>
        <WindowAction Name="MyCustomClose" Icon="@IconName.Close" OnClick="@MyCustomCloseHandler" />
    </WindowActions>
</TelerikWindow>

@code {
    bool isVisible { get; set; }
    string result { get; set; }

    void VisibleChangedHandler(bool currVisible)
    {
        // if you don't do this, the window won't close because of the user action
        // so if you don't update the view-model here, the window will not close when Esc is pressed

        //isVisible = currVisible;

        result = $"the window is now visible: {isVisible}";

        Console.WriteLine($"Closing because of Esc key");
    }

    void MyCustomCloseHandler()
    {
        //will fire on click only
        isVisible = false;
        //since the built-in window UI didn't invoke this change, the event will not fire
    }

    public void ToggleWindow()
    {
        isVisible = !isVisible;

        result = $"the window is now visible: {isVisible}";
    }
}