Autocompletion doesn't work in front of other text

Autocompletion for e.g. emoji doesn't work if it's started in front of other text.

**Expected behavior**
The same behavior as when autocompletion is invoked at the end of a line:

**Actual behavior**
No autocompletion list is shown.

Reproduce the issue by invoking emoji autocompletion before existing text, then compare it with invocation at the end of a line. Trace the frontend autocompletion entry point and verify that the suggestion list appears in both positions without changing the expected results.

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