Avoid board change during compilation/upload

By threading the boardChange callback we can busy wait until
the compilation/upload phase has ended and change the board when done.

Fixes #6035

app/src/processing/app/Base.java

@@ -1422,7 +1422,7 @@ public void rebuildRecentBoardsMenu() throws Exception {
     boardMenuScroller.setTopFixedCount(3 + index);
   }
 
-  public void onBoardOrPortChange() {
+  public synchronized void onBoardOrPortChange() {
     BaseNoGui.onBoardOrPortChange();
 
     // reload keywords when package/platform changes
@@ -1632,7 +1632,20 @@ private JRadioButtonMenuItem createBoardMenusAndCustomMenus(
     @SuppressWarnings("serial")
     Action action = new AbstractAction(board.getName()) {
       public void actionPerformed(ActionEvent actionevent) {
-        selectTargetBoard((TargetBoard) getValue("b"));
+        new Thread()
+        {
+            public void run() {
+              if (activeEditor != null && activeEditor.isUploading()) {
+                  // block until isUploading becomes false, but aboid blocking the UI
+                  while (activeEditor.isUploading()) {
+                    try {
+                      Thread.sleep(100);
+                    } catch (InterruptedException e) {}
+                  }
+              }
+              selectTargetBoard((TargetBoard) getValue("b"));
+            }
+        }.start();
       }
     };
     action.putValue("b", board);