fix(glfw): report a zero framebuffer size so the no-drawable guard is reachable

adapters/glfw/glfw_host.cpp

@@ -103,6 +103,12 @@ void Host::close() noexcept {
   glfwMakeContextCurrent(nullptr);
   glfwTerminate();
   have_pointer_ = false;
+  // A closed host reports no size: a second run() must not see the previous
+  // window's framebuffer size before its first refresh_sizes().
+  fb_width_ = 0;
+  fb_height_ = 0;
+  fb_scale_x_ = 1.0f;
+  fb_scale_y_ = 1.0f;
 }
 
 bool Host::run(const RunConfig& cfg, const FrameFn& frame, Status* st) {
@@ -172,7 +178,17 @@ bool Host::run(const RunConfig& cfg, const FrameFn& frame, Status* st) {
 
     if (fb_width_ == 0 || fb_height_ == 0) {
       // Minimised, or a size the compositor reports as zero: nothing to draw
-      // into. Idle briefly rather than spin, so an exit deadline still lands.
+      // into, and nothing to hand `begin_frame` -- a stale larger size would be
+      // a lie about the framebuffer. Idle on events rather than spin, and still
+      // honour the exit deadline here: a bounded run that never gets a drawable
+      // must end, not idle past the deadline it was given. There is no frame to
+      // dump in this state, so the screenshot is skipped.
+      if (exit_after_s > 0.0 && glfwGetTime() - start >= exit_after_s) {
+        std::fprintf(stderr,
+                     "[glfw] exit deadline reached with no drawable (framebuffer 0x0);"
+                     " nothing to dump\n");
+        break;
+      }
       glfwWaitEventsTimeout(0.05);
       continue;
     }
@@ -224,8 +240,13 @@ void Host::refresh_sizes() noexcept {
   glfwGetWindowContentScale(window_, &sx, &sy);
   content_scale_x_ = sx;
   content_scale_y_ = sy;
-  if (fw > 0) fb_width_ = static_cast<u32>(fw);
+  // Assigned unconditionally, zero included: an iconified window really does
-  if (fh > 0) fb_height_ = static_cast<u32>(fh);
+  // report a 0x0 framebuffer (Windows, and several Linux window managers), and
+  // the loop's no-drawable guard reads exactly this. Only ever writing a
+  // positive size would leave the guard unreachable and keep handing
+  // `begin_frame` a stale size for a framebuffer that is no longer that big.
+  fb_width_ = fw > 0 ? static_cast<u32>(fw) : 0u;
+  fb_height_ = fh > 0 ? static_cast<u32>(fh) : 0u;
   if (ww > 0 && fw > 0) fb_scale_x_ = static_cast<f32>(fw) / static_cast<f32>(ww);
   if (wh > 0 && fh > 0) fb_scale_y_ = static_cast<f32>(fh) / static_cast<f32>(wh);
 }