diff --git a/primer-miso/src/Primer/Miso.hs b/primer-miso/src/Primer/Miso.hs
index d13aa7ba1..b804a9b85 100644
--- a/primer-miso/src/Primer/Miso.hs
+++ b/primer-miso/src/Primer/Miso.hs
@@ -154,12 +154,14 @@ import Primer.Miso.Util (
   bindingsInExpr,
   bindingsInType,
   clayToMiso,
+  exitFullscreen,
   findASTDef,
   kindsInType,
   nodeSelectionType,
   optToName,
   readMs,
   realToClay,
+  requestFullscreen,
   runMutationWithNullDb,
   runTC,
   selectedDefName,
@@ -333,7 +335,14 @@ updateModel =
     SetEvalOpts f -> do
       #components % #eval % #opts %= f
       setEval
-    ToggleFullscreenEval -> #components % #eval % #fullscreen %= not
+    ToggleFullscreenEval -> do
+      #components % #eval % #fullscreen %= not
+      fs' <- use $ #components % #eval % #fullscreen
+      scheduleIO_ $
+        void
+          if fs'
+            then requestFullscreen
+            else exitFullscreen
   where
     -- TODO the only part of this that should really require `IO` is writing to a database
     -- (currently we use `NullDb` anyway but this will change)
diff --git a/primer-miso/src/Primer/Miso/Util.hs b/primer-miso/src/Primer/Miso/Util.hs
index 7d84ef015..8a08540e1 100644
--- a/primer-miso/src/Primer/Miso/Util.hs
+++ b/primer-miso/src/Primer/Miso/Util.hs
@@ -7,6 +7,8 @@
 
 -- | Things which should really be upstreamed rather than living in this project.
 module Primer.Miso.Util (
+  requestFullscreen,
+  exitFullscreen,
   startAppWithSavedState,
   showMs,
   readMs,
@@ -57,6 +59,7 @@ import Data.Map qualified as Map
 import Data.String (String)
 import Data.UUID.Types qualified as UUID
 import GHC.Base (error)
+import Language.Javascript.JSaddle qualified as JS
 import Linear (Additive, R1 (_x), R2 (_y), V2, zero)
 import Linear.Affine (Point (..), unP)
 import Miso (
@@ -134,6 +137,11 @@ import Primer.TypeDef (TypeDefMap)
 import Primer.Typecheck (ExprT, exprTtoExpr, typeTtoType)
 import StmContainers.Map qualified as StmMap
 
+requestFullscreen :: JSM JS.JSVal
+requestFullscreen = JS.eval ("document.documentElement.requestFullscreen()" :: MisoString)
+exitFullscreen :: JSM JS.JSVal
+exitFullscreen = JS.eval ("document.exitFullscreen()" :: MisoString)
+
 {- Miso -}
 
 -- https://github.com/dmjio/miso/issues/749