Even back in 2.80 for the ctrl+M to work in renaming a marker you have to have the mouse cursor over the marker region. So the timeline and dopesheet editors have the header region(Top of editor by default), the player or playhead region(Technically the playhead is the blue bubble with the current frame number. the reset is the frame numbers.), The main region(Where you see the keyframes), the channels region(the area to the left with the search box and names of the keyed items and groupings of the keyed items), The sidebar(on the right side. I call this the n-panel, because the hotkey to open/close it is N), and marker region(Where the markers are seen at the bottom or above the header if you have the header flipped to bottom).
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