Cursor jumps to start of doc when typing immediately after layout change

## What Happened
When switching between two xkb keyboard sources (tried 'gb'/'fr' and 'gb'/'ru') the cursor jumps to the start of the document and the previously typed character is lost if a character is typed too rapidly after pressing the layout-change shortcut (used `` and ``)

## Expected Behavior
The layout should change and the the appropriate character entered into the text at the current position.

## Steps to Reproduce
1. Set up two keyboard layouts using System Settings (I also removed any ibus methods)
2. Open a new document in Code and enter several blank lines.
3. Repeatly alternate a key that generates a different character in the two layouts with a layout-change shortcut.
4. After one or two layout switches the text entry point jumps to the start of the document.

## Platform Information
Elementary 6.0 in VirtualBox VM.
It does not seem to happen when Code is run in Ubuntu Focal (Gnome) so may not be a Code issue. Please move issue as required if confirmed to occur with Elementary 6.0 on bare metal.

Reproduce the cursor jump in Code on Elementary 6.0 using the reported gb/fr or gb/ru layouts and the listed shortcuts. Compare behavior in Ubuntu Focal, then determine whether the failure is in Code or the Elementary/GTK keyboard-input environment; done means the responsible component and a reliable reproduction are identified.

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