Search field in 'Quickly Add to Project' fails with Chinese Pinyin input unless manually focused

A licensee reported that under the Chinese language setting in UE5, using the “Quickly Add to Project” button results in incorrect or ignored input when entering text via Pinyin IME. If the user starts typing without manually focusing the field, the input is misinterpreted. Only the first character is registered, and the rest is ignored or rendered incorrectly. This does not happen with English input, or if the user clicks the field before typing.

Upon investigation, the root cause appears to be related to the SMultiBoxWidget, which does not fully activate IME input context when focus is set programmatically. This was confirmed by setting a breakpoint in the SMultiBoxWidget::OnFilterTextChanged method and observing that it only fires once during IME input unless the field is manually focused. In comparison, the Place Actors window (still available in UE5 and using SSearchBox) handles IME input correctly.

This behavior has been verified in UE 5.4, UE 5.5, UE 5.6 Preview, and Main branch (CL 42589677). In UE 5.3.2, although no input field is shown visually, typing still triggers the same issue, indicating that MultiBox is used under the hood.

As a workaround, Chinese input users can rely on the Place Actors window, which does not exhibit this problem.

1. On Windows 11, ensure the Chinese (Simplified, Mainland China) language is installed and set up. If not already available, add it via Settings > Time & Language > Language & Region > Add a language.
2. Switch the system input method to Chinese (Pinyin).
2. Create a blank project with UE5.
3. Click the “Quickly Add to Project” button (top toolbar).
4. Without manually clicking the search input field, begin typing Pinyin input (e.g., "da").
5. Observe the output and how many characters are processed.
6. Now repeat the same steps, but this time manually click on the search input field to focus it before typing.

Expected Result:
Typing using Pinyin IME should correctly produce the composed Chinese character (e.g., "大" for "da"), and allow full IME input handling without needing to manually click the field.

Actual Result:
If the user does not manually focus the field first, only the first keypress is registered (e.g., “d”), followed by partial or broken input (e.g., “d大d” or similar). The issue does not occur when using an English input method, nor when clicking the input field before typing.