My workaround would be a macro which clicks a location, unfortunately there are multiple sizes. The non-pausing ones go away after a while, which also expires while paused. So I usually pause to read and voila. Esc could be a good candidate imo if this turns into a suggestion. Maybe (big) Enter but so many keys are used between safety systems and special scripts (numpad enter is Sifa ack).
BTW is it possible to show scenario text window again after it has been closed? Sometimes I want to reread it.
If the scenario author has ticked the checkbox for that, yes. It will then be available via the letter symbol in the F4 HUD. Otherwise, no.
Sorry for the late reply, but IIRC Shift+Backspace closes message windows. Just be really sure you're holding shift, otherwise it applies emergency braking instead!