Multiple terminals per tab, rearrangeable by drag or keyboard

A tab is now a view holding an ordered list of panes rather than a single
terminal. New panes open side by side; moving one against a top or bottom
edge restacks the view, and against a side edge returns it to a row.

Layout is a flat list with one orientation per view rather than a split
tree. That covers a shell beside an agent without the structure a tree
needs, and it can be replaced once mixed layouts actually matter.

Panes live in a GtkBox and are reordered in place, so rearranging never
unparents a terminal and running processes and scrollback survive the move.

Dragging a pane uses its header as the handle so it never competes with the
terminal's own mouse handling, and drops go through the same moveRelative
path as the Ctrl+Shift+arrow shortcuts.

Also fixes an ordering bug this surfaced: View.create used to add its first
pane immediately, firing the title callback into a Tab that had not been
initialized yet. The view is now created empty and the caller adds the pane
once it has finished wiring itself up.

src/Terminal.zig

@@ -9,6 +9,7 @@
 const std = @import("std");
 const cairo = @import("cairo");
 const gdk = @import("gdk");
+const gobject = @import("gobject");
 const gtk = @import("gtk");
 const pango = @import("pango");
 const pangocairo = @import("pangocairo");
@@ -29,9 +30,6 @@ const font_spec = "monospace 11";
 /// Padding between the grid and the widget edge, in pixels.
 const pad: f64 = 8;
 
-/// Corner rounding of the terminal pane.
-const corner_radius: f64 = 10;
-
 alloc: std.mem.Allocator,
 session: *Session,
 
@@ -49,6 +47,10 @@ run_buf: std.ArrayListUnmanaged(u8) = .empty,
 /// Called when the session's title changes, so the owner can retitle the tab.
 on_title: *const fn (ctx: ?*anyopaque, title: []const u8) void,
 on_exit: *const fn (ctx: ?*anyopaque) void,
+
+/// Called when this terminal takes keyboard focus.
+on_focus: *const fn (ctx: ?*anyopaque) void,
+
 ctx: ?*anyopaque = null,
 
 pub fn create(
@@ -56,6 +58,7 @@ pub fn create(
     cbs: struct {
         on_title: *const fn (ctx: ?*anyopaque, title: []const u8) void,
         on_exit: *const fn (ctx: ?*anyopaque) void,
+        on_focus: *const fn (ctx: ?*anyopaque) void,
         ctx: ?*anyopaque,
     },
 ) !*Terminal {
@@ -72,6 +75,7 @@ pub fn create(
         .font = font,
         .on_title = cbs.on_title,
         .on_exit = cbs.on_exit,
+        .on_focus = cbs.on_focus,
         .ctx = cbs.ctx,
     };
 
@@ -129,6 +133,16 @@ pub fn create(
     );
     w.addController(click.as(gtk.EventController));
 
+    // Watching the property rather than just the click gesture means focus
+    // taken programmatically or by keyboard navigation is reported too.
+    _ = gobject.Object.signals.notify.connect(
+        area,
+        *Terminal,
+        &onNotifyHasFocus,
+        self,
+        .{ .detail = "has-focus" },
+    );
+
     return self;
 }
 
@@ -175,6 +189,14 @@ fn onSessionTitle(ctx: ?*anyopaque, title: []const u8) void {
     self.on_title(self.ctx, title);
 }
 
+fn onNotifyHasFocus(
+    _: *gtk.DrawingArea,
+    _: *gobject.ParamSpec,
+    self: *Terminal,
+) callconv(.c) void {
+    if (self.widget().hasFocus() != 0) self.on_focus(self.ctx);
+}
+
 fn onSessionExit(ctx: ?*anyopaque) void {
     const self: *Terminal = @ptrCast(@alignCast(ctx.?));
     self.on_exit(self.ctx);
@@ -313,27 +335,17 @@ fn drawFunc(
     };
 }
 
-fn render(self: *Terminal, cr: *cairo.Context, width: c_int, height: c_int) !void {
+fn render(self: *Terminal, cr: *cairo.Context, _: c_int, _: c_int) !void {
     const term = &self.session.term;
     const screen = term.screens.active;
 
-    // Background. Clipped to a rounded rectangle so the terminal reads as an
-    // inset pane next to the sidebar, the way Zen insets web content.
+    // Background. The pane frame around us clips to its own rounded corners,
+    // so this just fills.
     const default_bg: theme.Rgb = if (term.colors.background.get()) |c|
         .from(c)
     else
         theme.bg;
     {
-        roundedRect(
-            cr,
-            0,
-            0,
-            @floatFromInt(width),
-            @floatFromInt(height),
-            corner_radius,
-        );
-        cr.clip();
-
         const r, const g, const b = default_bg.cairoRgb();
         cr.setSourceRgb(r, g, b);
         cr.paint();
@@ -547,19 +559,6 @@ fn drawCursor(
     }
 }
 
-/// Trace a rounded rectangle as the current path.
-fn roundedRect(cr: *cairo.Context, x: f64, y: f64, w: f64, h: f64, r: f64) void {
-    const radius = @min(r, @min(w, h) / 2);
-    const pi = std.math.pi;
-
-    cr.newSubPath();
-    cr.arc(x + w - radius, y + radius, radius, -pi / 2.0, 0);
-    cr.arc(x + w - radius, y + h - radius, radius, 0, pi / 2.0);
-    cr.arc(x + radius, y + h - radius, radius, pi / 2.0, pi);
-    cr.arc(x + radius, y + radius, radius, pi, 3.0 * pi / 2.0);
-    cr.closePath();
-}
-
 /// Resolve a cell's style into concrete colors and attributes.
 fn appearance(
     pin: vt.Pin,