core: set focused surface pointer to null if matches on delete

We previously never set the focused pointer to null. I thought this
would be fine because a `hasSurface` check would say it doesn't exist.
But I didn't account for the fact that a deleted surface followed very
quickly by a new surface would free the pointer, then the allocation
would reuse the very same pointer, making `hasSurface` return a false
positive.

Well, technically, hasSurface is not wrong, the surface exists, but its
not really the same surface, its just a surface that happens to have the
same pointer as a previously freed surface.

Co-authored-by: Will Pragnell <wpragnell@gmail.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/src/App.zig b/src/App.zig
index c7721eb54..942581e3e 100644
--- a/src/App.zig
+++ b/src/App.zig
@@ -139,6 +139,16 @@ pub fn addSurface(self: *App, rt_surface: *apprt.Surface) !void {
 /// Delete the surface from the known surface list. This will NOT call the
 /// destructor or free the memory.
 pub fn deleteSurface(self: *App, rt_surface: *apprt.Surface) void {
+    // If this surface is the focused surface then we need to clear it.
+    // There was a bug where we relied on hasSurface to return false and
+    // just let focused surface be but the allocator was reusing addresses
+    // after free and giving false positives, so we must clear it.
+    if (self.focused_surface) |focused| {
+        if (focused == &rt_surface.core_surface) {
+            self.focused_surface = null;
+        }
+    }
+
     var i: usize = 0;
     while (i < self.surfaces.items.len) {
         if (self.surfaces.items[i] == rt_surface) {