fix: WidgetIter will also return first widget

Also includes some miscelaneous cleanups in WidgetIter.

README.md

@@ -46,7 +46,7 @@ If you'd like to suggest a new feature or design change, please open an issue fi
   - [ ] Tabbed Pane
   - [ ] Split Pane
   - [ ] Scrolled Pane
-- [ ] Focus Stack
+- [ ] Focus System
 - [x] Theming
   - [ ] Built-in themes
 - [ ] Layout overflow handling

src/WidgetIter.zig

@@ -1,27 +1,39 @@
-//! This is a iterator to iterate over the widget tree
-//! Depth-first
+//! This is a iterator to iterate over the widget tree using a depth-first search algorithm.
 const std = @import("std");
 
 const Widget = @import("widget.zig").Widget;
 
-current: *Widget,
+/// The widget the iterator is currently on.
+/// This is usually the widget just returned by the next function,
+/// although this field should also be used for initializing the iterator,
+/// in which case it is returned from next once.
+w: *Widget,
+
+/// Only true when the iterator is first initialized to return self.w instead of the next widget.
+first: bool = true,
 
 const WidgetIter = @This();
 
+/// Returns the next widget in the tree.
 pub fn next(self: *WidgetIter) ?*Widget {
+    if (self.first) {
+        self.first = false;
+        return self.w;
+    }
+
     var last: ?*Widget = null;
     while (true) {
-        const children = self.current.children();
+        const children = self.w.children();
         const slice = if (last) |l|
             children[std.mem.indexOfScalar(*Widget, children, l).? + 1 ..]
         else
             children;
         if (slice.len == 0) {
-            last = self.current;
-            self.current = self.current.data.parent orelse return null;
+            last = self.w;
+            self.w = self.w.data.parent orelse return null;
         } else {
-            self.current = slice[0];
-            return self.current;
+            self.w = slice[0];
+            return slice[0];
         }
     }
 }