Is it possible somehow to search across all open tabs/sessions in iTerm2? I'd like to do something like a regular search (Cmd+F), searching the scrollback buffer, but across everything I have open instead of just the current tab/session.
In a window menu, go to
Windows -> Exposé All Tabs. This will show you all open iTerm windows on one screen.
All iTerm2 tabs will be shown tiled on the main screen. You can mouse over a tab to see it larger, and clicking on it will restore the windows and select that tab and bring its window to the front. You can search the contents of all tabs by typing in the search field that appears on the left. Making a selection from the results below it will highlight the tab that contains that text. This is useful when you have many tabs open and can't find the one you're looking for.