Difficult to edit a token amount when it is <0.0001

[elena-zh]

Related to #1281

1. Select a tiny amount of a Buy token to get a Sell token amount <0.0001 (or vice versa)
2. Try to edit the Sell token amount

AR: not possible until delete the whole value

ER: should be possible to edit a value without removing it first

[alfetopito]

For future (me), solving this issue also fixes #1330

[elena-zh]

To add: impossible to edit such amount in a wallet integrated browser

Skype_Video.mp4

[stale]

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[elena-zh]

Still reproducible