diff --git a/backend/src/SDLBackend.zig b/backend/src/SDLBackend.zig
index 9d795a2f7..a7989b44f 100644
--- a/backend/src/SDLBackend.zig
+++ b/backend/src/SDLBackend.zig
@@ -17,9 +17,9 @@ const dvui = @import("dvui");
 const GpuRenderer = @import("GpuRenderer.zig");
 pub const viewport_map = @import("viewport_map.zig");
 const TouchSlots = @import("touch_slots.zig");
+const glass_look = @import("glass_look");
 /// The OS windows besides the main one an app draws with (`Viewport`).
-pub const glass_look = @import("glass_look");
-const viewports = @import("viewports.zig");
+pub const viewports = @import("viewports.zig");
 /// dvui's floating windows as OS windows of their own from the one frame (`floatingWindowOs`).
 pub const os_windows = @import("os_windows.zig");
 /// What this backend keeps count of as it runs (`health`).
diff --git a/backend/src/viewports_none.zig b/backend/src/viewports_none.zig
index 87a205714..84f73859a 100644
--- a/backend/src/viewports_none.zig
+++ b/backend/src/viewports_none.zig
@@ -4,9 +4,12 @@
 //! null, so an app's floats, menus and dialogs stay in the main window.
 //!
 //! An app that builds on either kind of backend imports this beside it and takes the backend's
-//! own where it has one:
+//! own where its build linked the backend package (fizzy passes that as a build option):
 //!
-//!     const viewports = if (@hasDecl(backend, "viewports")) backend.viewports else @import("viewports_none");
+//!     const viewports = if (linked_fizzy_backend) backend.viewports else @import("viewports_none");
+//!
+//! Not `@hasDecl(backend, "viewports")`: a declaration that loses its `pub` is invisible to it,
+//! and the app would quietly keep every float in its main window.
 const dvui = @import("dvui");
 
 /// A window's Liquid Glass this frame, as the backend package's (`platform.window.WindowGlassLook`):
diff --git a/build/exe.zig b/build/exe.zig
index 39ddba83d..d64f5d813 100644
--- a/build/exe.zig
+++ b/build/exe.zig
@@ -265,6 +265,10 @@ pub fn addFizzyExecutableForTarget(
     exe.root_module.addImport("platform", platformModule(b, native, resolved_target, optimize, macos_sdl_paths));
     // On dvui's own SDL3 backend (`-Dnative-backend=sdl3`), which has no viewports.
     exe.root_module.addImport("viewports_none", fizzy_backend.viewportsNoneModule(backendDependency(b), dvui_mod));
+    // Which of the two the executable links (`backend_native.viewports`).
+    const native_backend_opts = b.addOptions();
+    native_backend_opts.addOption(bool, "fizzy", native_backend == .fizzy);
+    exe.root_module.addImport("native_backend", native_backend_opts.createModule());
 
     // The glass look the backend dresses OS windows with; `core` takes the same module.
     const glass_look = fizzy_backend.glassLookModule(backendDependency(b));
diff --git a/src/backend/backend_native.zig b/src/backend/backend_native.zig
index a80137de4..82f8aa961 100644
--- a/src/backend/backend_native.zig
+++ b/src/backend/backend_native.zig
@@ -150,7 +150,9 @@ pub fn windowGlass(win: *dvui.Window, look: WindowGlassLook) bool {
 /// OS windows besides the main one, each showing a part of the one frame — a float popped out
 /// (`plans/POPOUT_WINDOWS_PLAN.md`): the backend package's, for any dvui app on it. On dvui's own
 /// SDL3 backend (`-Dnative-backend=sdl3`) there are none, as on the web, and floats stay in.
-pub const viewports = if (@hasDecl(@import("backend"), "viewports")) @import("backend").viewports else @import("viewports_none");
+/// Chosen by the backend the build linked, not by looking for the declaration: a `viewports` the
+/// backend stops exporting fails the build here instead of quietly keeping every float in.
+pub const viewports = if (@import("native_backend").fizzy) @import("backend").viewports else @import("viewports_none");
 
 /// Fizzy keeps the window's geometry in `layout.zon`, beside its regions — one file for where the
 /// window and everything in it were left, and the file it has always kept the frame in.