Simplify the opening editor behaviour again

This partially reverts 16fce5577d.

src/wine-host/bridges/vst2.h

@@ -33,6 +33,13 @@
 #include "../editor.h"
 #include "../utils.h"
 
+/**
+ * A marker struct to indicate that the editor is about to be opened.
+ *
+ * @see Vst2Bridge::editor
+ */
+struct EditorOpening {};
+
 /**
  * This hosts a Windows VST2 plugin, forwards messages sent by the Linux VST
  * plugin and provides host callback function for the plugin to talk back.
@@ -75,7 +82,9 @@ class Vst2Bridge {
      * but in our approach there will be an opportunity to handle events in
      * between these two calls. Most plugins will handle this just fine, but
      * some plugins end up blocking indefinitely while waiting for the other
-     * function to be called, hence why this function is needed.
+     * function to be called, hence why this function is needed. For
+     * individually hosted plugins this check is done implicitely in
+     * `Vst2Bridge::handle_win32_events()`.
      */
     bool should_skip_message_loop();
 
@@ -261,17 +270,25 @@ class Vst2Bridge {
      * Wine window, and embedding that Wine window into a window provided by the
      * host. Should be empty when the editor is not open.
      *
-     * There is some special behavior with regards to message handling when the
-     * editor is in the process of being opened, see
-     * `should_postpone_message_loop()`.
+     * This field can have three possible states:
+     *
+     * - `std::nullopt` when the editor is closed.
+     * - An `Editor` object when the editor is open.
+     * - `EditorOpening` when the editor is not yet open, but the host has
+     *   already called `effEditGetRect()` and is about to call `effEditOpen()`.
+     *   This is needed because there is a race condition in some bugs that
+     *   cause them to crash or enter an infinite Win32 message loop when
+     *   `effEditGetRect()` gets dispatched and we then enter the message loop
+     *   loop before `effEditOpen()` gets called. Most plugins will handle this
+     *   just fine, but a select few plugins make the assumption that the editor
+     *   is already open once `effEditGetRect()` has been called, even if
+     *   `effEditOpen` has not yet been dispatched. VST hsots on Windows will
+     *   call these two events in sequence, so the bug would never occur there.
+     *   To work around this we'll use this third state to temporarily stop
+     *   processing Windows events in the one or two ticks between these two
+     *   events.
      *
      * @see should_postpone_message_loop
      */
-    std::optional<Editor> editor;
-
-    /**
-     * Keeps track of when the editor is being opened during the two-phase
-     * process of the host calling `effEditOpen()` and `effEditGetRect()`.
-     */
-    bool editor_is_opening = false;
+    std::variant<std::monostate, Editor, EditorOpening> editor;
 };