optimize title and menubar redraw

src/Window.zig

@@ -183,13 +183,29 @@ fn onTabBar(self: *Window) bool {
     return out.state().layout == .tabbed;
 }
 
-fn setString(self: *Window, field: *?[]u8, value: ?[*:0]const u8) void {
+/// Replace an owned string field, returning whether the value actually
+/// changed. Clients re-send strings that have not moved — a terminal animating
+/// a spinner in its title re-sets the same text several times a second — and
+/// the caller can use the answer to skip the IPC broadcast and the tab bar
+/// redraw those no-op updates would otherwise trigger.
+fn setString(self: *Window, field: *?[]u8, value: ?[*:0]const u8) bool {
     const gpa = self.wm.gpa;
+    const new: ?[]const u8 = if (value) |v| std.mem.span(v) else null;
+
+    const same = if (field.*) |old|
+        if (new) |n| std.mem.eql(u8, old, n) else false
+    else
+        new == null;
+    if (same) return false;
+
     if (field.*) |old| gpa.free(old);
     field.* = null;
-    if (value) |v| {
-        field.* = gpa.dupe(u8, std.mem.span(v)) catch null;
+    if (new) |n| {
+        // A failed dupe leaves the field null, which compares unequal to the
+        // next event carrying the same text, so the copy is retried then.
+        field.* = gpa.dupe(u8, n) catch null;
     }
+    return true;
 }
 
 fn onEvent(_: *river.WindowV1, event: river.WindowV1.Event, self: *Window) void {
@@ -227,23 +243,25 @@ fn onEvent(_: *river.WindowV1, event: river.WindowV1.Event, self: *Window) void
         },
 
         .app_id => |ev| {
-            self.setString(&self.app_id, ev.app_id);
-            self.applyRules();
-            self.wm.ipcDirty();
+            if (self.setString(&self.app_id, ev.app_id)) {
+                self.applyRules();
+                self.wm.ipcDirty();
+            }
         },
 
         .title => |ev| {
-            self.setString(&self.title, ev.title);
-            self.applyRules();
-            self.wm.ipcDirty();
-            // The tab bar has this title painted into it, so a window that is
-            // wearing a tab needs the strip redrawn. Terminals retitle
-            // themselves constantly, so ask only when it will show.
-            if (self.onTabBar()) self.wm.needsManage();
+            if (self.setString(&self.title, ev.title)) {
+                self.applyRules();
+                self.wm.ipcDirty();
+                // The tab bar has this title painted into it, so a window that
+                // is wearing a tab needs the strip redrawn. Terminals retitle
+                // themselves constantly, so ask only when it will show.
+                if (self.onTabBar()) self.wm.needsManage();
+            }
         },
 
         .identifier => |ev| {
-            self.setString(&self.identifier, ev.identifier);
+            _ = self.setString(&self.identifier, ev.identifier);
         },
 
         .parent => |ev| {