Root: fix crash on output disable

If there is a transaction inflight when an output is disabled then we
must call View.commitTransaction() for any views evacuated from the
disabled output to ensure transaction state remains consistent.

Root.commitTransaction() will not call View.commitTransaction()
for these evacuated views as their output is no longer around.

 river/Root.zig | 11 +++++++++--
 river/View.zig |  5 +++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/river/Root.zig b/river/Root.zig
index 5743d30..7399408 100644
--- a/river/Root.zig
+++ b/river/Root.zig
@@ -280,6 +280,10 @@ pub fn deactivateOutput(root: *Self, output: *Output) void {
 
             view.inflight_wm_stack_link.remove();
             view.inflight_wm_stack_link.init();
+
+            if (view.inflight_transaction) {
+                view.commitTransaction();
+            }
         }
     }
     // Use the first output in the list as fallback. If the last real output
@@ -552,11 +556,16 @@ fn sendConfigures(root: *Self) void {
     while (output_it.next()) |output| {
         var focus_stack_it = output.inflight.focus_stack.iterator(.forward);
         while (focus_stack_it.next()) |view| {
+            assert(!view.inflight_transaction);
+            view.inflight_transaction = true;
+
             // This can happen if a view is unmapped while a layout demand including it is inflight
+            // If a view has been unmapped, don't send it a configure.
             if (!view.mapped) continue;
 
             if (view.configure()) {
                 root.inflight_configures += 1;
+
                 view.saveSurfaceTree();
                 view.sendFrameDone();
             }
@@ -617,8 +626,6 @@ fn commitTransaction(root: *Self) void {
 
             view.tree.node.reparent(root.hidden.tree);
             view.popup_tree.node.reparent(root.hidden.tree);
-
-            view.commitTransaction();
         }
     }
 
diff --git a/river/View.zig b/river/View.zig
index 4de5fc1..b5b418e 100644
--- a/river/View.zig
+++ b/river/View.zig
@@ -131,6 +131,8 @@ popup_tree: *wlr.SceneTree,
 constraints: Constraints = .{},
 
 mapped: bool = false,
+/// This is true if the View is involved in the currently inflight transaction.
+inflight_transaction: bool = false,
 /// This indicates that the view should be destroyed when the current
 /// transaction completes. See View.destroy()
 destroying: bool = false,
@@ -273,6 +275,9 @@ pub fn resizeUpdatePosition(view: *Self, width: i32, height: i32) void {
 }
 
 pub fn commitTransaction(view: *Self) void {
+    assert(view.inflight_transaction);
+    view.inflight_transaction = false;
+
     view.foreign_toplevel_handle.update();
 
     // Tag and output changes must be applied immediately even if the configure sequence times out.