sixel: implement private mode 80, DECSDM (sixel scrolling mode)
DECSDM - Sixel Scrolling Mode
Sixel Scrolling Mode
You can set the sixel scrolling mode by using the Sixel Scrolling feature in the Graphics Set-Up screen. You can also select this mode by using the sixel display mode (DECSDM) control function.
Sixel Scrolling Enabled
When sixel display mode is enabled, the sixel active position begins at the upper-left corner of the ANSI text active position. Scrolling occurs when the sixel active position reaches the bottom margin of the graphics page. When sixel mode is exited, the text cursor is set to the current sixel cursor position.
The VT300 sends a sixel next line (-) character following a sixel dump. The top line of the sixel image may scroll off the screen if (1) your application returns the sixel dump to the terminal, or (2) you perform a sixel dump to a video terminal connected to the VT300 printer port.
NOTE: You can prevent the sixel image from scrolling off the screen by disabling the sixel scrolling feature.
Sixel Scrolling Disabled
When sixel scrolling is disabled, the sixel active position begins at the upper-left corner of the active graphics page. The terminal ignores any commands that attempt to advance the active position below the bottom margin of the graphics page. When sixel mode is exited, the text cursor does not change from the position it was in when sixel mode was entered.
Foot currently implements the enabled behavior. The default in XTerm is set.
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