B/N shortcuts move focus in background while field editors are open

[microbit-robert]

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Description

The B/N keys move focus in the background while field editors are open. This is more noticeable on the screen reader branch due to the noises (end of program and nesting changes). These shortcuts should not be handled while field editors are open.

Does this affect page up/down, and home/end shortcuts?

Reproduction steps

1. Go to https://raspberrypifoundation.github.io/blockly-keyboard-experimentation/screenreader/
2. Select the simple circle program
3. Open the colour picker field editor
4. Hit the B / N keys to move to the previous and next stack as appropriate
5. Focus is moving in the background, this is more noticeable in the screen reader branch because sounds are triggered
6. Hit escape to close the field editor
7. Observed that focus has moved somewhere unexpected

Stack trace

Screenshots

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Browsers

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