layout/master: new windows goes into master and gets focus

README.md

@@ -182,8 +182,11 @@ separate setting, `repeat`, under [Input](#input).
 column is divided, so the bottom edge always lands exactly on the output edge.
 
 **monocle** — every window gets the full area; only the most recently focused
-one is rendered. A window spawned into monocle or tabbed takes focus as it
-maps, so it comes up in front instead of hidden behind the current one.
+one is rendered.
+
+In every layout a newly mapped window goes to the front of the arrangement
+order — the first master slot — and takes focus, as in dwm. A window spawned
+onto a tag nobody is viewing leaves both the order and focus alone.
 
 **tabbed** — same geometry as monocle, minus a strip at the top where att_wm
 draws one solid colour block per window, the focused one highlighted. The blocks

src/Wm.zig

@@ -602,23 +602,27 @@ fn assignOutputs(self: *Wm) void {
     }
 }
 
-/// Hand a freshly mapped window the newest focus serial, and focus it, when it
-/// arrives into a stacking layout.
+/// Put a freshly mapped window at the front of its output's arrangement order
+/// and focus it.
 ///
-/// Those layouts render only the most recently focused window, so a new window
-/// that keeps its initial serial of zero is hidden the instant it appears and
-/// cannot even be reached with the pointer. Making it the top of the stack
-/// without also moving focus would be worse still — the keyboard would go on
-/// talking to a window that is no longer on screen — so the two move together,
-/// which is what dwm does for every new client anyway.
-///
-/// In master a new window is visible wherever it lands in the order, so focus
-/// is left where the user put it.
+/// This is dwm's attach() plus focus(): a new client lands in master rather
+/// than at the bottom of the stack, and the keyboard follows it there. In a
+/// stacking layout the move is not optional — those layouts render only the
+/// most recently focused window, so a window that kept its initial serial of
+/// zero would be hidden the instant it appeared and unreachable even with the
+/// pointer, and promoting it without moving focus would leave the keyboard
+/// talking to a window that is no longer on screen.
 ///
 /// Runs before `arrangeAll` so the serial set here is the one the layout picks
-/// its top window with.
+/// its top window with, and the order set here is the one it arranges.
 fn focusNewWindows(self: *Wm) void {
-    for (self.windows.items) |win| {
+    // `attachToMaster` reorders `windows`, so re-read the list each time round
+    // rather than holding a slice across the move. It only ever moves a window
+    // to a lower index, so nothing after `i` shifts and no new window is
+    // skipped.
+    var i: usize = 0;
+    while (i < self.windows.items.len) : (i += 1) {
+        const win = self.windows.items[i];
         if (!win.wants_initial_focus) continue;
         if (win.closed or !win.mapped) continue;
         win.wants_initial_focus = false;
@@ -627,11 +631,12 @@ fn focusNewWindows(self: *Wm) void {
         if (win.floating or win.fullscreen) continue;
 
         const out = win.output orelse continue;
-        if (!layout.stacks(out.state().layout)) continue;
         // Spawned onto a tag nobody is looking at: nothing to come to the
         // front of, and stealing focus would yank the user off their tag.
         if ((win.tags & out.tags) == 0) continue;
 
+        self.attachToMaster(win, out);
+
         self.focus_serial += 1;
         win.focus_serial = self.focus_serial;
         for (self.seats.items) |seat| {
@@ -640,6 +645,20 @@ fn focusNewWindows(self: *Wm) void {
     }
 }
 
+/// Move `win` ahead of every other tiled window on `out`, so it takes the
+/// first master slot. Windows on other outputs and on other tags keep their
+/// relative order.
+fn attachToMaster(self: *Wm, win: *Window, out: *Output) void {
+    for (self.windows.items) |other| {
+        if (other == win) return; // already the first tiled window here
+        if (other.output != out) continue;
+        if ((other.tags & out.tags) == 0) continue;
+        if (other.floating or other.fullscreen) continue;
+        self.moveToFront(win, other);
+        return;
+    }
+}
+
 /// If a seat's focused window is no longer visible, move focus the way dwm
 /// does: to the most recently focused visible window on the same output.
 fn validateFocus(self: *Wm, seat: *Seat) void {

src/layout.zig

@@ -56,16 +56,6 @@ pub const Result = struct {
     stacked: bool = false,
 };
 
-/// Whether a layout puts every window in the same place, so only the top one
-/// is rendered. `arrange` reports the same thing after the fact; this answers
-/// it for callers that need to know before the geometry is computed.
-pub fn stacks(layout: Layout) bool {
-    return switch (layout) {
-        .master => false,
-        .monocle, .tabbed => true,
-    };
-}
-
 /// Fill `cells` with one rectangle per window, in arrangement order.
 ///
 /// Each cell is the *outer* rectangle including space for the border; the

src/test.zig

@@ -102,13 +102,12 @@ test "monocle gives every window the full area and reports stacked" {
     for (cells) |cell| try testing.expectEqual(area, cell);
 }
 
-test "stacks agrees with what arrange reports" {
+test "master does not report stacked" {
     var cells: [2]Box = undefined;
     var p = params(1, 0.55);
     p.tabbar_height = 22;
-    for ([_]layout.Layout{ .master, .monocle, .tabbed }) |mode| {
-        try testing.expectEqual(layout.arrange(mode, p, &cells).stacked, layout.stacks(mode));
-    }
+
+    try testing.expect(!layout.arrange(.master, p, &cells).stacked);
 }
 
 test "tabbed reserves the bar strip above the windows" {