Clipper: Assert on extraneous calls to Step(). (#4822) + Demo tweak.

imgui.h

@@ -1422,7 +1422,7 @@ enum ImGuiKey_
     ImGuiKey_GamepadRStickLeft,     // [Analog]
     ImGuiKey_GamepadRStickRight,    // [Analog]
 
-    // Keyboard Modifiers
+    // Keyboard Modifiers (explicitly submitted by backend via AddKeyEvent() calls)
     // - This is mirroring the data also written to io.KeyCtrl, io.KeyShift, io.KeyAlt, io.KeySuper, in a format allowing
     //   them to be accessed via standard key API, allowing calls such as IsKeyPressed(), IsKeyReleased(), querying duration etc.
     // - Code polling every keys (e.g. an interface to detect a key press for input mapping) might want to ignore those
@@ -1430,11 +1430,9 @@ enum ImGuiKey_
     // - In theory the value of keyboard modifiers should be roughly equivalent to a logical or of the equivalent left/right keys.
     //   In practice: it's complicated; mods are often provided from different sources. Keyboard layout, IME, sticky keys and
     //   backends tend to interfere and break that equivalence. The safer decision is to relay that ambiguity down to the end-user...
-    ImGuiKey_ModCtrl,
-    ImGuiKey_ModShift,
-    ImGuiKey_ModAlt,
-    ImGuiKey_ModSuper,
+    ImGuiKey_ModCtrl, ImGuiKey_ModShift, ImGuiKey_ModAlt, ImGuiKey_ModSuper,
 
+    // End of list
     ImGuiKey_COUNT,                 // No valid ImGuiKey is ever greater than this value
 
     // [Internal] Prior to 1.87 we required user to fill io.KeysDown[512] using their own native index + a io.KeyMap[] array.