Auto merge of #24577 - paulrouget:transientThrobber, r=jdm

Add a loading indicator in transient mode

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #24549 (GitHub issue number if applicable)

support/hololens/ServoApp/BrowserPage.cpp

@@ -28,14 +28,17 @@ void BrowserPage::BindServoEvents() {
     forwardButton().IsEnabled(forward);
   });
   servoControl().OnLoadStarted([=] {
-    throbber().IsActive(true);
+    urlbarLoadingIndicator().IsActive(true);
+    transientLoadingIndicator().IsIndeterminate(true);
+
     reloadButton().IsEnabled(false);
     reloadButton().Visibility(Visibility::Collapsed);
     stopButton().IsEnabled(true);
     stopButton().Visibility(Visibility::Visible);
   });
   servoControl().OnLoadEnded([=] {
-    throbber().IsActive(false);
+    urlbarLoadingIndicator().IsActive(false);
+    transientLoadingIndicator().IsIndeterminate(false);
     reloadButton().IsEnabled(true);
     reloadButton().Visibility(Visibility::Visible);
     stopButton().IsEnabled(false);
@@ -65,6 +68,8 @@ void BrowserPage::SetTransientMode(bool transient) {
   servoControl().SetTransientMode(transient);
   navigationBar().Visibility(transient ? Visibility::Collapsed
                                        : Visibility::Visible);
+  transientLoadingIndicator().Visibility(transient ? Visibility::Visible
+                                            : Visibility::Collapsed);
 }
 
 void BrowserPage::SetArgs(hstring args) { servoControl().SetArgs(args); }

support/hololens/ServoApp/BrowserPage.xaml

@@ -83,6 +83,7 @@
     <Grid.RowDefinitions>
       <RowDefinition Height="auto"/>
       <RowDefinition Height="*"/>
+      <RowDefinition Height="auto"/>
     </Grid.RowDefinitions>
     <Grid Grid.Row="0" x:Name="navigationBar" Background="{ThemeResource InkToolbarButtonBackgroundThemeBrush}">
       <Grid.ColumnDefinitions>
@@ -105,8 +106,9 @@
         </Button>
       </StackPanel>
       <TextBox Text="" IsTabStop="true" InputScope="Url" PlaceholderText="Type a URL" x:Name="urlTextbox" Grid.Column="1" KeyUp="OnURLEdited" IsSpellCheckEnabled="False" Margin="3,0"/>
-      <ProgressRing x:Name="throbber" Grid.Column="2" Margin="10,0"/>
+      <ProgressRing x:Name="urlbarLoadingIndicator" Grid.Column="2" Margin="10,0"/>
     </Grid>
     <local:ServoControl TabIndex="0" x:Name="servoControl" Grid.Row="1"/>
+    <ProgressBar x:Name="transientLoadingIndicator" Visibility="Collapsed" Grid.Row="2"/>
   </Grid>
 </Page>