Reproduction: https://blazorrepl.telerik.com/QJEJlAvY33zlToJO18.

To reproduce:

step 1: observe that cells div.k-spreadsheet-fixed-container fills div.k-spreadsheet-view
step 2: Move the splitter handler to left
step 3: Observe that the div.k-spreadsheet-view width is adjusted to the new size of its parent, but div.k-spreadsheet-fixed-container is not.

Expected:  Spreadsheet component handles updating the size its parts.

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ADMIN EDIT

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A possible workaround for the time being is to simulate the browser resize event - the Spreadsheet is properly resized in this case. 

Here is how to achieve that: https://blazorrepl.telerik.com/wzOfQbEW38JtEjFD36.

I want to change the ColumnsCount property value at runtime, for example, based on the Excel files that the app is loading.

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TELERIK EDIT:

A possible workaround is to recreate the Spreadsheet component. The downside will be a minor UI flicker. Alternatively, set ColumnsCount to a small value like 1. In this way the component will display only the existing columns in the opened file.

<TelerikNumericTextBox @bind-Value="@SpreadsheetColumnsCount"
                       Min="0" Max="999"
                       Width="80px" />

<TelerikButton OnClick="@OnButtonClick">Apply ColumnsCount</TelerikButton>

@if (SpreadsheetVisible)
{
    <TelerikSpreadsheet Data="@SpreadsheetData"
                        ColumnsCount="@SpreadsheetColumnsCount">
    </TelerikSpreadsheet>
}

@code {
    private byte[]? SpreadsheetData { get; set; }

    private int SpreadsheetColumnsCount { get; set; } = 10;

    private bool SpreadsheetVisible { get; set; } = true;

    private async Task OnButtonClick()
    {
        SpreadsheetVisible = false;
        await Task.Delay(1);

        SpreadsheetVisible = true;
    }
}