Triple click in TextBox does not select all text

* .NET Core Version: All
* Windows version: 1903
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

**Problem description:**
Triple clicking in a `TextBox` does not select all text in the `TextBox`.

**Actual behavior:**
Triple clicking in a `TextBox` does not select all text in the `TextBox`.

**Expected behavior:**
Triple clicking in a `TextBox` should select all text.

**Minimal repro:**
- Create a window containing a `TextBox`
- Write some text into the `TextBox`
- Triple click with the mouse