@uiryuu I tried to address your review feedback. Please have a look.
@svobs I know you are busy, but if you get a chance look please have a look.

In my testing these key bindings matched:

• seek 20
• seek -20 exact
• seek 20 relative+exact
• seek 20 exact+relative
• seek 20 relative exact

These did not match (as expected):

• seek 20 keyframes
• seek 20 relative+keyframes
• seek 20 relative keyframes
• seek 20 absolute

The keyframe flag can't be supported without dealing with the current code confusion over SeekOption.relative. That value is labeled in the UI as Keyframe seek in the Seek type setting under Control. That makes sense. But internally it is named relative. The code where it is used in PlayerCore.seek looks like this:

    case .relative:
      mpv.command(.seek, args: ["\(relativeSecond)", "relative"], checkError: false)

Trouble is, if you really want to do a keyframe seek you must pass the keyframes flag. Although the seek command will default to keyframes that default can be changed. From the mpv manual entry for hr-seek:

--hr-seek=<no|absolute|yes|default>
Select when to use precise seeks that are not limited to keyframes. Such seeks require decoding video from the previous keyframe up to the target position and so can take some time depending on decoding performance. For some video formats, precise seeks are disabled. This option selects the default choice to use for seeks; it is possible to explicitly override that default in the definition of key bindings and in input commands.

• no: Never use precise seeks.
• absolute: Use precise seeks if the seek is to an absolute position in the file, such as a chapter seek, but not for relative seeks like the default behavior of arrow keys.
• default: Like absolute, but enable hr-seeks in audio-only cases. The exact behavior is implementation specific and may change with new releases (default).
• yes: Use precise seeks whenever possible.
  always: Same as yes (for compatibility).

So if you have set hr-seek=yes then you must pass the keyframes flag in a seek command if you want to seek by keyframe.

As the issue at hand is about getting exact to work I don't think we need to address this in the current PR.