TextBox: Double-clicking the empty area after text does not select the closest word

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*This content was largely generated by AI. AI makes mistakes.*

### Describe the bug

Double-clicking the empty area after the text in a WinUI `TextBox` does not select any text. The expected behavior is to select the closest word, rather than requiring the pointer to be directly over the word.

### Why is this important?

Users expect to use double-click to select text for editing. Ignoring double-clicks in the unused portion of a text field makes selection less convenient and inconsistent with the behavior requested in the related UniGetUI report below.

### Steps to reproduce the bug

1. Display a `TextBox` that is wide enough to leave empty space after its text.
2. Enter `Hello world`.
3. Double-click the empty area to the right of `world`, inside the `TextBox`.

This is a minimal scenario for the reported behavior; a standalone reproduction project has not been attached.

### Actual behavior

No text is selected when double-clicking the empty area.

### Expected behavior

Select the closest word, which is `world` in this example, as if the user had double-clicked that word directly.

### Screenshots

Not provided.

### NuGet package version

Not yet recorded.

### Windows version

Not yet recorded.

### Additional context

The same symptom was reported in [UniGetUI #4160: Allow double clicking empty space in searchbox](https://github.com/Devolutions/UniGetUI/issues/4160).

That report specifically requests selecting the closest word when double-clicking the empty area after the text. The UniGetUI maintainer [closed it as not planned on October 5, 2025, attributing the behavior to WinUI](https://github.com/Devolutions/UniGetUI/issues/4160#issuecomment-3369188731). This issue tracks the behavior in the WinUI repository.

Start with a minimal WinUI TextBox reproduction using the reported steps, comparing a double-click directly on “world” with one in the empty area after it. Confirm the expected closest-word selection and inspect the relevant TextBox pointer and selection behavior; no source file or test is named in the report, and done means the empty-area double-click selects “world”.

Written by the indexing model from the issue text.