libghostty: expose row-local render selections

Render state already tracks the selected cell range for each viewport row,
but C renderers could only get the full terminal selection. That required
consumers to map global selection pins back into row-local spans themselves.

Add row selection data to the render-state row API. Querying the new row
data returns GHOSTTY_NO_VALUE for unselected rows and writes the inclusive
start and end columns for selected rows. The render example now demonstrates
setting a selection and reading the row-local range while iterating rows.

example/c-vt-render/README.md

@@ -2,8 +2,8 @@
 
 This contains an example of how to use the `ghostty-vt` render-state API
 to create a render state, update it from terminal content, iterate rows
-and cells, read styles and colors, inspect cursor state, and manage dirty
-tracking.
+and cells, read styles and colors, inspect cursor and row-local selection
+state, and manage dirty tracking.
 
 This uses a `build.zig` and `Zig` to build the C program so that we
 can reuse a lot of our build logic and depend directly on our source

example/c-vt-render/src/main.c

@@ -46,6 +46,32 @@ int main(void) {
   ghostty_terminal_vt_write(
       terminal, (const uint8_t*)content, strlen(content));
 
+  // Select "underlined" on the second row. Render state exposes this
+  // later as a row-local selected cell range.
+  GhosttyGridRef selection_start = GHOSTTY_INIT_SIZED(GhosttyGridRef);
+  GhosttyPoint selection_start_pt = {
+      .tag = GHOSTTY_POINT_TAG_ACTIVE,
+      .value = { .coordinate = { .x = 0, .y = 1 } },
+  };
+  result = ghostty_terminal_grid_ref(
+      terminal, selection_start_pt, &selection_start);
+  assert(result == GHOSTTY_SUCCESS);
+
+  GhosttyGridRef selection_end = GHOSTTY_INIT_SIZED(GhosttyGridRef);
+  GhosttyPoint selection_end_pt = {
+      .tag = GHOSTTY_POINT_TAG_ACTIVE,
+      .value = { .coordinate = { .x = 9, .y = 1 } },
+  };
+  result = ghostty_terminal_grid_ref(terminal, selection_end_pt, &selection_end);
+  assert(result == GHOSTTY_SUCCESS);
+
+  GhosttySelection selection = GHOSTTY_INIT_SIZED(GhosttySelection);
+  selection.start = selection_start;
+  selection.end = selection_end;
+  result = ghostty_terminal_set(
+      terminal, GHOSTTY_TERMINAL_OPT_SELECTION, &selection);
+  assert(result == GHOSTTY_SUCCESS);
+
   result = ghostty_render_state_update(render_state, terminal);
   assert(result == GHOSTTY_SUCCESS);
   //! [render-state-update]
@@ -154,6 +180,18 @@ int main(void) {
     printf("Row %2d [%s]: ", row_index,
            row_dirty ? "dirty" : "clean");
 
+    // Query the row-local selection range. Rows without a selection return
+    // GHOSTTY_NO_VALUE; selected rows return inclusive start/end columns.
+    GhosttyRenderStateRowSelection row_selection =
+        GHOSTTY_INIT_SIZED(GhosttyRenderStateRowSelection);
+    result = ghostty_render_state_row_get(
+        row_iter, GHOSTTY_RENDER_STATE_ROW_DATA_SELECTION, &row_selection);
+    assert(result == GHOSTTY_SUCCESS || result == GHOSTTY_NO_VALUE);
+    if (result == GHOSTTY_SUCCESS) {
+      printf("selection=%u..%u ",
+             row_selection.start_x, row_selection.end_x);
+    }
+
     // Get cells for this row (reuses the same cells handle).
     result = ghostty_render_state_row_get(
         row_iter, GHOSTTY_RENDER_STATE_ROW_DATA_CELLS, &cells);

include/ghostty/vt/render.h

@@ -221,6 +221,9 @@ typedef enum GHOSTTY_ENUM_TYPED {
    *  valid as long as the underlying render state is not updated. 
    *  It is unsafe to use cell data after updating the render state. */
   GHOSTTY_RENDER_STATE_ROW_DATA_CELLS = 3,
+
+  /** Row-local selected cell range (GhosttyRenderStateRowSelection). */
+  GHOSTTY_RENDER_STATE_ROW_DATA_SELECTION = 4,
   GHOSTTY_RENDER_STATE_ROW_DATA_MAX_VALUE = GHOSTTY_ENUM_MAX_VALUE,
 } GhosttyRenderStateRowData;
 
@@ -235,6 +238,29 @@ typedef enum GHOSTTY_ENUM_TYPED {
   GHOSTTY_RENDER_STATE_ROW_OPTION_MAX_VALUE = GHOSTTY_ENUM_MAX_VALUE,
 } GhosttyRenderStateRowOption;
 
+/**
+ * Row-local selection range.
+ *
+ * This struct uses the sized-struct ABI pattern. Initialize with
+ * GHOSTTY_INIT_SIZED(GhosttyRenderStateRowSelection) before querying
+ * GHOSTTY_RENDER_STATE_ROW_DATA_SELECTION.
+ *
+ * Querying GHOSTTY_RENDER_STATE_ROW_DATA_SELECTION returns GHOSTTY_NO_VALUE
+ * if the current row does not intersect the current selection.
+ *
+ * @ingroup render
+ */
+typedef struct {
+  /** Size of this struct in bytes. Must be set to sizeof(GhosttyRenderStateRowSelection). */
+  size_t size;
+
+  /** Start column of the row-local selection range, inclusive. */
+  uint16_t start_x;
+
+  /** End column of the row-local selection range, inclusive. */
+  uint16_t end_x;
+} GhosttyRenderStateRowSelection;
+
 /**
  * Render-state color information.
  *

src/terminal/c/render.zig

@@ -31,6 +31,7 @@ const RowIteratorWrapper = struct {
     /// These are the raw pointers into the render state data.
     raws: []const page.Row,
     cells: []const std.MultiArrayList(renderpkg.RenderState.Cell),
+    selection: []const ?[2]size.CellCountInt,
     dirty: []bool,
 
     /// The color palette from the render state, needed to resolve
@@ -61,6 +62,13 @@ pub const RowCells = ?*RowCellsWrapper;
 /// C: GhosttyRenderStateDirty
 pub const Dirty = renderpkg.RenderState.Dirty;
 
+/// C: GhosttyRenderStateRowSelection
+pub const RowSelection = extern struct {
+    size: usize = @sizeOf(RowSelection),
+    start_x: u16 = 0,
+    end_x: u16 = 0,
+};
+
 /// C: GhosttyRenderStateCursorVisualStyle
 pub const CursorVisualStyle = enum(c_int) {
     bar = 0,
@@ -241,6 +249,7 @@ fn getTyped(
                 .y = null,
                 .raws = row_data.items(.raw),
                 .cells = row_data.items(.cells),
+                .selection = row_data.items(.selection),
                 .dirty = row_data.items(.dirty),
                 .palette = &state.state.colors.palette,
             };
@@ -381,6 +390,7 @@ pub fn row_iterator_new(
         .y = undefined,
         .raws = undefined,
         .cells = undefined,
+        .selection = undefined,
         .dirty = undefined,
         .palette = undefined,
     };
@@ -564,6 +574,7 @@ pub const RowData = enum(c_int) {
     dirty = 1,
     raw = 2,
     cells = 3,
+    selection = 4,
 
     /// Output type expected for querying the data of the given kind.
     pub fn OutType(comptime self: RowData) type {
@@ -572,6 +583,7 @@ pub const RowData = enum(c_int) {
             .dirty => bool,
             .raw => row.CRow,
             .cells => RowCells,
+            .selection => RowSelection,
         };
     }
 };
@@ -654,6 +666,14 @@ fn rowGetTyped(
                 .palette = it.palette,
             };
         },
+        .selection => {
+            const out_size = out.size;
+            if (out_size < @sizeOf(RowSelection)) return .invalid_value;
+
+            const sel = it.selection[y] orelse return .no_value;
+            out.start_x = sel[0];
+            out.end_x = sel[1];
+        },
     }
 
     return .success;
@@ -845,6 +865,7 @@ test "render: row iterator new/free" {
     try testing.expectEqual(@as(?size.CellCountInt, null), iterator_ptr.y);
     try testing.expectEqual(row_data.items(.raw).len, iterator_ptr.raws.len);
     try testing.expectEqual(row_data.items(.cells).len, iterator_ptr.cells.len);
+    try testing.expectEqual(row_data.items(.selection).len, iterator_ptr.selection.len);
     try testing.expectEqual(row_data.items(.dirty).len, iterator_ptr.dirty.len);
 }
 
@@ -1026,6 +1047,60 @@ test "render: row get/set dirty" {
     try testing.expect(!dirty);
 }
 
+test "render: row get selection" {
+    var terminal: terminal_c.Terminal = null;
+    try testing.expectEqual(Result.success, terminal_c.new(
+        &lib.alloc.test_allocator,
+        &terminal,
+        .{
+            .cols = 10,
+            .rows = 3,
+            .max_scrollback = 10_000,
+        },
+    ));
+    defer terminal_c.free(terminal);
+
+    const t = terminal.?.terminal;
+    const screen = t.screens.active;
+    try screen.select(.init(
+        screen.pages.pin(.{ .active = .{ .x = 2, .y = 1 } }).?,
+        screen.pages.pin(.{ .active = .{ .x = 4, .y = 1 } }).?,
+        false,
+    ));
+
+    var state: RenderState = null;
+    try testing.expectEqual(Result.success, new(
+        &lib.alloc.test_allocator,
+        &state,
+    ));
+    defer free(state);
+
+    try testing.expectEqual(Result.success, update(state, terminal));
+
+    var it: RowIterator = null;
+    try testing.expectEqual(Result.success, row_iterator_new(
+        &lib.alloc.test_allocator,
+        &it,
+    ));
+    defer row_iterator_free(it);
+
+    try testing.expectEqual(Result.success, get(state, .row_iterator, @ptrCast(&it)));
+
+    var sel: RowSelection = .{};
+    try testing.expect(row_iterator_next(it));
+    try testing.expectEqual(Result.no_value, row_get(it, .selection, @ptrCast(&sel)));
+
+    try testing.expect(row_iterator_next(it));
+    sel = .{};
+    try testing.expectEqual(Result.success, row_get(it, .selection, @ptrCast(&sel)));
+    try testing.expectEqual(@as(u16, 2), sel.start_x);
+    try testing.expectEqual(@as(u16, 4), sel.end_x);
+
+    try testing.expect(row_iterator_next(it));
+    sel = .{};
+    try testing.expectEqual(Result.no_value, row_get(it, .selection, @ptrCast(&sel)));
+}
+
 test "render: row iterator next" {
     var terminal: terminal_c.Terminal = null;
     try testing.expectEqual(Result.success, terminal_c.new(