Fix Sheet native blocking issue by adding a conditional background blocker.

1. In `Sheet.java`: Add a transparent `Button` (blocker) to the Sheet container if `Form.activePeerCount > 0`. This intercepts pointer events for native peers.
2. Ensure blocker is removed if peers are no longer present.
3. Update `KotlinUiTest.kt` (lightweight only) and `BrowserComponentScreenshotTest.java` (with peer) to include Sheet display, verifying both scenarios.
4. Use `sheet.show(0)` in tests to prevent animation-related instability.

Author: google-labs-jules[bot]

CodenameOne/src/com/codename1/ui/Sheet.java

@@ -495,7 +495,8 @@ public void paint(Graphics g, Rectangle rect) {
                 blocker.putClientProperty("SheetBlocker", Boolean.TRUE);
                 blocker.setUIID("Container");
                 blocker.getAllStyles().setBgTransparency(0);
-                blocker.setPreferredSize(new com.codename1.ui.geom.Dimension(10000, 10000));
+                int size = Math.max(CN.getDisplayWidth(), CN.getDisplayHeight()) * 2;
+                blocker.setPreferredSize(new com.codename1.ui.geom.Dimension(size, size));
                 ((Button) blocker).addActionListener(new ActionListener() {
                     @Override
                     public void actionPerformed(ActionEvent evt) {

scripts/hellocodenameone/common/src/main/kotlin/com/codenameone/examples/hellocodenameone/tests/KotlinUiTest.kt

@@ -12,6 +12,7 @@ import com.codename1.ui.Slider
 import com.codename1.ui.TextArea
 import com.codename1.ui.TextField
 import com.codename1.ui.layouts.BoxLayout
+import com.codename1.ui.util.UITimer
 
 class KotlinUiTest : BaseTest() {
     override fun runTest(): Boolean {