[SPARK-35117][UI] Change progress bar back to highlight ratio of tasks in progress

### What changes were proposed in this pull request?
Small UI update to add highlighting the number of tasks in progress in a stage/job instead of highlighting the whole in progress stage/job. This was the behavior pre Spark 3.1 and the bootstrap 4 upgrade.

### Why are the changes needed?

To add back in functionality lost between 3.0 and 3.1. This provides a great visual queue of how much of a stage/job is currently being run.

### Does this PR introduce _any_ user-facing change?

Small UI change.

Before:
![image](https://user-images.githubusercontent.com/3536454/115216189-3fddaa00-a0d2-11eb-88e0-e3be925c92f0.png)

After (and pre Spark 3.1):
![image](https://user-images.githubusercontent.com/3536454/115216216-48ce7b80-a0d2-11eb-9953-2adb3b377133.png)

### How was this patch tested?

Updated existing UT.

Closes apache#32214 from Kimahriman/progress-bar-started.

Authored-by: Adam Binford <[email protected]>
Signed-off-by: Kousuke Saruta <[email protected]>

Author: Kimahriman

core/src/main/resources/org/apache/spark/ui/static/webui.css

@@ -113,7 +113,7 @@ table.sortable td {
   box-shadow: inset 1px 0 0 rgba(0,0,0,.15), inset 0 -1px 0 rgba(0,0,0,.15);
 }
 
-.progress.progress-started {
+.progress .progress-bar.progress-started {
   background-color: #A0DFFF;
   background-image: -moz-linear-gradient(top, #A4EDFF, #94DDFF);
   background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#A4EDFF), to(#94DDFF));
@@ -124,7 +124,7 @@ table.sortable td {
   filter: progid:dximagetransform.microsoft.gradient(startColorstr='#FFA4EDFF', endColorstr='#FF94DDFF', GradientType=0);
 }
 
-.progress .progress-bar {
+.progress .progress-bar.progress-completed {
   background-color: #3EC0FF;
   background-image: -moz-linear-gradient(top, #44CBFF, #34B0EE);
   background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#44CBFF), to(#34B0EE));

core/src/main/scala/org/apache/spark/ui/UIUtils.scala

@@ -461,13 +461,14 @@ private[spark] object UIUtils extends Logging {
       skipped: Int,
       reasonToNumKilled: Map[String, Int],
       total: Int): Seq[Node] = {
-    val ratio = if (total == 0) 100.0 else (completed.toDouble/total)*100
+    val ratio = if (total == 0) 100.0 else (completed.toDouble / total) * 100
     val completeWidth = "width: %s%%".format(ratio)
     // started + completed can be > total when there are speculative tasks
     val boundedStarted = math.min(started, total - completed)
-    val startWidth = "width: %s%%".format((boundedStarted.toDouble/total)*100)
+    val startRatio = if (total == 0) 0.0 else (boundedStarted.toDouble / total) * 100
+    val startWidth = "width: %s%%".format(startRatio)
 
-    <div class={ if (started > 0) s"progress progress-started" else s"progress" }>
+    <div class="progress">
       <span style="text-align:center; position:absolute; width:100%;">
         {completed}/{total}
         { if (failed == 0 && skipped == 0 && started > 0) s"($started running)" }
@@ -478,7 +479,8 @@ private[spark] object UIUtils extends Logging {
           }
         }
       </span>
-      <div class="progress-bar" style={completeWidth}></div>
+      <div class="progress-bar progress-completed" style={completeWidth}></div>
+      <div class="progress-bar progress-started" style={startWidth}></div>
     </div>
   }
 

core/src/test/scala/org/apache/spark/ui/UIUtilsSuite.scala

@@ -113,7 +113,8 @@ class UIUtilsSuite extends SparkFunSuite {
   test("SPARK-11906: Progress bar should not overflow because of speculative tasks") {
     val generated = makeProgressBar(2, 3, 0, 0, Map.empty, 4).head.child.filter(_.label == "div")
     val expected = Seq(
-      <div class="progress-bar" style="width: 75.0%"></div>
+      <div class="progress-bar progress-completed" style="width: 75.0%"></div>,
+      <div class="progress-bar progress-started" style="width: 25.0%"></div>
     )
     assert(generated.sameElements(expected),
       s"\nRunning progress bar should round down\n\nExpected:\n$expected\nGenerated:\n$generated")