copying in backend jumps to other tab

when I try to copy text from one of the email templates in abc/settings/email-settings by mouse, the tab is always changing when I release the mouse button.

another error: I have only one room for rent. So I have only one calendar.
when making a new booking manually in the backend, the field for “persons” is showing “choose calendar” meanwhile in calendar-field it’s showing “choose”.
And sometimes the calendar-field is not showing “choose” but showing my calendar-title directly. In this case, I can’t choose any person number as this field is blocked -> what means that I can’t enter a new booking. When I clean the cache, I get the “choose”-title in calendar-field and then can choose the number of persons, too…