1 files changed, 27 insertions, 0 deletions

diff --git a/src/pardes.zig b/src/pardes.zig
index dcf0bf55..1009d13d 100644
--- a/src/pardes.zig
+++ b/src/pardes.zig
@@ -3548,6 +3548,8 @@ pub const Event = union(enum) {
     /// The host pointer left the drawable area. Unlike an out-of-range motion,
     /// this must not clamp onto and preview the final grid cell.
     pointer_leave,
+    /// The window gained (true) or lost the host's keyboard focus.
+    focus: bool,
     tick,
 };
 
@@ -4322,6 +4324,10 @@ pub const Pardes = struct {
     /// A press a terminal's program took (mouse.programMouse): its drags
     /// and release are the program's too, wherever the pointer goes.
     mouse_grab: ?mouse.Grab = null,
+    /// Whether the host window has the keyboard (Event.focus), and the pane
+    /// whose program was last told it has it (reportFocus).
+    window_focused: bool = true,
+    focus_told: ?struct { id: usize, serial: u32 } = null,
     hover_col: u16 = 0,
     hover_row: u16 = 0,
     pointer_raw_col: u16 = 0,
@@ -5007,6 +5013,25 @@ pub const Pardes = struct {
     }
 
     /// The column the keyboard is in: its tag's, or its pane's.
+    /// DECSET 1004: a program that asked is told `CSI I` when its pane
+    /// takes the keyboard (the active pane, its body, the window focused)
+    /// and `CSI O` when it gives it up, to another pane, a tag or header,
+    /// or another window.
+    fn reportFocus(p: *Pardes) void {
+        const want: @FieldType(Pardes, "focus_told") = blk: {
+            if (!p.window_focused or p.header_focus) break :blk null;
+            const pane = p.panes[p.active] orelse break :blk null;
+            if (pane.focus != .body) break :blk null;
+            break :blk .{ .id = p.active, .serial = pane.serial };
+        };
+        if (std.meta.eql(want, p.focus_told)) return;
+        if (p.focus_told) |old| if (p.panes[old.id]) |pane| {
+            if (pane.serial == old.serial and panes.terminal.reportsFocus(pane)) p.emitWrite(old.id, "\x1b[O");
+        };
+        if (want) |new| if (panes.terminal.reportsFocus(p.panes[new.id].?)) p.emitWrite(new.id, "\x1b[I");
+        p.focus_told = want;
+    }
+
     pub fn noteTyped(p: *Pardes) void {
         if (p.header_focus) {
             if (p.header_column) |c| p.setActiveColumn(c);
@@ -5653,6 +5678,7 @@ pub const Pardes = struct {
     }
 
     pub fn update(p: *Pardes, ev: Event) void {
+        defer p.reportFocus();
         // A tick with nothing animating is the one event that cannot change
         // the screen.
         p.needs_frame = p.needs_frame or ev != .tick;
@@ -5785,6 +5811,7 @@ pub const Pardes = struct {
             .command => |line| _ = p.executeBuiltinLine(p.active, line),
             .pinch => |scale| p.ov_pinch_scale = scale,
             .touch_scroll => |delta| p.ov_touch_scroll_delta = delta,
+            .focus => |focused| p.window_focused = focused,
             .pointer_leave => {
                 p.pointer_inside = false;
                 p.pointer_body_hit = null;