Background: In the framework, the position of the caret rect is updated on each cursor position change such that if the user initiates composing input, the current cursor position can be used for the first character until the composing rect can be sent.

Previously, no update was sent on selection changes, on the assumption that the most recent cursor position will remain the correct position for the duration of the selection. While this is the case for forward selections, it is an incorrect assumption for reversed selections, where selection.base > selection.extent.

We now update the cursor position during selection changes such that the cursor position sent to the embedder is always the position at which next text input would occur. This is the start position of the selection or min(selection.baseOffset, selection.extentOffset).

Issue: https://github.com/flutter/flutter/issues/137677