Fix two crashes

One was where WM was assuming that a seat existed during first manage,
but that's not always true, so we have to check that before running the
initialization code. I also split that off into its own function like in
Window.

The other crash was when trying to calculate the layout with the
output's width and/or height equal to zero, it would crash subtracting
the border width.

I discovered both of these when try to restart beansprout without
restarting River.

src/Output.zig

@@ -576,8 +576,11 @@ pub fn manage(output: *Output) void {
         }
     }
 
-    // Calculate layout before managing windows
+    // Calculate layout before managing windows, but only if output dimensions are initialized
+    if (output.usable_geometry.width > 0 and output.usable_geometry.height > 0) {
         output.calculateLayout();
+    }
+
     var it = output.windows.iterator(.forward);
     while (it.next()) |window| {
         window.manage();
@@ -612,6 +615,8 @@ pub fn render(output: *Output) void {
 /// - Single window: maximized
 /// - Multiple windows: stack (45% left, vertically tiled), primary (55% right)
 fn calculateLayout(output: *Output) void {
+    // Shouldn't be called if height/width are not positive
+    assert(output.geometry.width > 0 and output.geometry.height > 0);
     // Get a list of active windows
     var active_list: DoublyLinkedList = .{};
     var active_count: u31 = 0;
@@ -714,8 +719,14 @@ fn calculateLayout(output: *Output) void {
         }
 
         // Make space for borders
+        if (window.pending_manage.dimensions.?.height > 2 * border_width and
+            window.pending_manage.dimensions.?.width > 2 * border_width)
+        {
             window.pending_manage.dimensions.?.height -= 2 * border_width;
             window.pending_manage.dimensions.?.width -= 2 * border_width;
+        } else {
+            log.warn("Can't add borders to some window; {s}'s dimensions are too small.", .{output.name orelse "some output"});
+        }
         window.pending_render.position.?.x += border_width;
         window.pending_render.position.?.y += border_width;
     }

src/WindowManager.zig

@@ -72,21 +72,17 @@ pub fn prevOutput(wm: *WindowManager, current: *Output) ?*Output {
     return @fieldParentPtr("link", prev_link);
 }
 
-fn manage_start(wm: *WindowManager) void {
-    const river_window_manager_v1 = wm.river_window_manager_v1;
-    const context = wm.context;
+fn initialize(wm: *WindowManager) void {
+    if (wm.context.initialized) return;
 
-    // This gets used shortly, so it goes at the beginning
-    context.manage();
-
-    if (!context.initialized) {
-        // This code runs during initial startup and after config reloads.
-        @branchHint(.cold);
-        context.initialized = true;
-
-        const seat = wm.seats.first() orelse @panic("Failed to get seat");
+    // We need a seat to intitialize this stuff, so let's just not do it right now.
+    // The WM can run fine without it, though, it won't be fully usuable.
+    const seat = wm.seats.first() orelse return;
     const river_seat_v1 = seat.river_seat_v1;
 
+    const context = wm.context;
+    context.initialized = true;
+
     // Tag bindings
     for (context.config.tag_binds.items) |tag_bind| {
         comptime var i: u8 = 1;
@@ -132,6 +128,19 @@ fn manage_start(wm: *WindowManager) void {
     }
 }
 
+fn manage(wm: *WindowManager) void {
+    const river_window_manager_v1 = wm.river_window_manager_v1;
+    const context = wm.context;
+
+    // This gets used shortly, so it goes at the beginning
+    context.manage();
+
+    if (!context.initialized) {
+        // This code runs during initial startup and after config reloads.
+        @branchHint(.cold);
+        wm.initialize();
+    }
+
     {
         var it = wm.outputs.iterator(.forward);
         while (it.next()) |output| {
@@ -147,7 +156,7 @@ fn manage_start(wm: *WindowManager) void {
     river_window_manager_v1.manageFinish();
 }
 
-fn render_start(wm: *WindowManager) void {
+fn render(wm: *WindowManager) void {
     const river_window_manager_v1 = wm.river_window_manager_v1;
     {
         var it = wm.seats.iterator(.forward);
@@ -171,8 +180,8 @@ fn windowManagerV1Listener(window_manager_v1: *river.WindowManagerV1, event: riv
         .unavailable => {
             fatal("Window manager unavailable (some other wm instance is running). Exiting", .{});
         },
-        .manage_start => wm.manage_start(),
-        .render_start => wm.render_start(),
+        .manage_start => wm.manage(),
+        .render_start => wm.render(),
         .output => |ev| {
             const output = Output.create(context, ev.id) catch @panic("Out of memory");
             wm.outputs.append(output);