Application of directional mode (SCP)
ECMA-48 is a bit fuzzy, or inconsistent, about how to apply SCP. It shall be applied to the current line immediately, but apparently not to another line when moving to it (e.g. with CUP)? If that’s the correct interpretation, some clarification would help.
In the current mintty implementation, it is furthermore interpreted like this: SCP is applied to the current line (as specified) so it is immediately updated on the screen; also SCP is set as a cursor property (dual behaviour); SCP mode is retained until another SCP is sent (except for terminal reset (soft/hard) which resets to default).
However, if the cursor moves (linefeed, CUP etc) the SCP mode is not applied immediately, but only when a character is actually written to the current position (this was more straight-forward to implement).
As an exception (also due to straight-forward implementation), writing a combining character only (e.g. directly after CUP) would not change the line property yet. Also, as this is a cursor property, it would be restored with a Restore Cursor operation (DECRC and related controls).
With this interpretation, I think it is not necessary to define or implement any special handling for other escape sequences (IL, EL, SU, ED, ...).