libghostty: add selection adjustment api

include/ghostty/vt/selection.h

@@ -77,6 +77,61 @@ typedef struct {
   bool rectangle;
 } GhosttySelection;
 
+/**
+ * Operation used to adjust a selection endpoint.
+ *
+ * Adjustment mutates the selection's logical end endpoint, not whichever
+ * endpoint is visually bottom/right. This preserves keyboard and drag
+ * behavior for both forward and reversed selections.
+ *
+ * @ingroup selection
+ */
+typedef enum GHOSTTY_ENUM_TYPED {
+  /** Move left to the previous non-empty cell, wrapping upward. */
+  GHOSTTY_SELECTION_ADJUST_LEFT = 0,
+
+  /** Move right to the next non-empty cell, wrapping downward. */
+  GHOSTTY_SELECTION_ADJUST_RIGHT = 1,
+
+  /**
+   * Move up one row at the current column, or to the beginning of the
+   * line if already at the top.
+   */
+  GHOSTTY_SELECTION_ADJUST_UP = 2,
+
+  /**
+   * Move down to the next non-blank row at the current column, or to the
+   * end of the line if none exists.
+   */
+  GHOSTTY_SELECTION_ADJUST_DOWN = 3,
+
+  /** Move to the top-left cell of the screen. */
+  GHOSTTY_SELECTION_ADJUST_HOME = 4,
+
+  /** Move to the right edge of the last non-blank row on the screen. */
+  GHOSTTY_SELECTION_ADJUST_END = 5,
+
+  /**
+   * Move up by one terminal page height, or to home if that would move
+   * past the top.
+   */
+  GHOSTTY_SELECTION_ADJUST_PAGE_UP = 6,
+
+  /**
+   * Move down by one terminal page height, or to end if that would move
+   * past the bottom.
+   */
+  GHOSTTY_SELECTION_ADJUST_PAGE_DOWN = 7,
+
+  /** Move to the left edge of the current line. */
+  GHOSTTY_SELECTION_ADJUST_BEGINNING_OF_LINE = 8,
+
+  /** Move to the right edge of the current line. */
+  GHOSTTY_SELECTION_ADJUST_END_OF_LINE = 9,
+
+  GHOSTTY_SELECTION_ADJUST_MAX_VALUE = GHOSTTY_ENUM_MAX_VALUE,
+} GhosttySelectionAdjust;
+
 /** @} */
 
 #ifdef __cplusplus

include/ghostty/vt/terminal.h

@@ -1123,6 +1123,35 @@ GHOSTTY_API GhosttyResult ghostty_terminal_get_multi(GhosttyTerminal terminal,
                                     void** values,
                                     size_t* out_written);
 
+/**
+ * Adjust a selection snapshot using terminal selection semantics.
+ *
+ * This mutates the caller-provided GhosttySelection in place. The logical end
+ * endpoint is always moved, regardless of whether the selection is forward or
+ * reversed visually. The input selection remains a snapshot: after adjustment,
+ * call ghostty_terminal_set() with GHOSTTY_TERMINAL_OPT_SELECTION to install it
+ * as the terminal-owned selection if desired.
+ *
+ * The selection's start and end grid refs must both be valid untracked
+ * snapshots for the given terminal's currently active screen. In practice,
+ * they must come from that terminal and screen, and no mutating terminal call
+ * may have occurred since the refs were produced or reconstructed from
+ * tracked refs. Passing refs from another terminal, another screen, or stale
+ * refs violates this precondition.
+ *
+ * @param terminal The terminal handle (NULL returns GHOSTTY_INVALID_VALUE)
+ * @param selection Selection snapshot to adjust in place
+ * @param adjustment The adjustment operation to apply
+ * @return GHOSTTY_SUCCESS on success, GHOSTTY_INVALID_VALUE if the terminal,
+ *         selection, selection references, or adjustment are invalid
+ *
+ * @ingroup terminal
+ */
+GHOSTTY_API GhosttyResult ghostty_terminal_selection_adjust(
+                                    GhosttyTerminal terminal,
+                                    GhosttySelection* selection,
+                                    GhosttySelectionAdjust adjustment);
+
 /**
  * Resolve a point in the terminal grid to a grid reference.
  *