I also face this issue when a popup is shown.

For example, I write an email with something like "see the file attached" and I do not attch the file. Thunderbird identifies there is a missing file and shows a popup asking if it is expected to send without attachment.

In this case,

- if grammalecte is deactivated, the cursor is visible everywhere
- if grammalecte is activated, the cursor is visible only over the popup