gh-138122: Improve bytecode panel (#142910)

The bytecode panel appears when a user generates a heatmap with
--opcodes and clicks the button to unfold a line and display the
bytecode instructions. Currently, an empty space appears on the
left where the line number, self, and total columns are displayed.
This area should instead extend those columns, rather than leaving
a gap.

Lib/profiling/sampling/_heatmap_assets/heatmap.css

@@ -1141,6 +1141,10 @@ @media (max-width: 600px) {
   .line-samples-cumulative {
     padding: 0 4px;
   }
+
+  .bytecode-panel {
+    margin: 8px 10px 8px 160px;
+  }
 }
 
 .bytecode-toggle {
@@ -1172,13 +1176,77 @@ .bytecode-spacer {
 }
 
 .bytecode-panel {
-  margin-left: 90px;
-  padding: 8px 15px;
-  background: var(--bg-secondary);
-  border-left: 3px solid var(--accent);
+  background: var(--bg-primary);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  box-shadow: var(--shadow-md);
   font-family: var(--font-mono);
   font-size: 12px;
-  margin-bottom: 4px;
+  color: var(--text-primary);
+  line-height: 1.5;
+  word-wrap: break-word;
+  overflow-wrap: break-word;
+  padding: 0;
+  margin: 8px 10px 8px 250px;
+  position: relative;
+  z-index: 1;
+  overflow-y: auto;
+  max-height: 500px;
+  flex: 1;
+  transition: padding 0.3s cubic-bezier(0.4, 0, 0.2, 1);
+}
+
+.bytecode-panel.expanded {
+  padding: 14px;
+}
+
+.bytecode-wrapper {
+  position: relative;
+  display: flex;
+  overflow: visible;
+  max-height: 0;
+  opacity: 0;
+  transition: max-height 0.4s cubic-bezier(0.4, 0, 0.2, 1), opacity 0.3s ease-in-out;
+}
+
+.bytecode-wrapper.expanded {
+  max-height: 600px;
+  opacity: 1;
+  transition: max-height 0.5s cubic-bezier(0.4, 0, 0.2, 1), opacity 0.4s ease-in-out;
+}
+
+/* Column backdrop matching table header columns (line/self/total) */
+.bytecode-columns {
+  display: none;
+  position: absolute;
+  left: 0;
+  overflow: hidden;
+  pointer-events: none;
+  z-index: 0;
+}
+
+.bytecode-wrapper.expanded .bytecode-columns {
+  display: flex;
+  top: 0;
+  bottom: 0;
+}
+
+.bytecode-panel::-webkit-scrollbar {
+  width: 8px;
+}
+
+.bytecode-panel::-webkit-scrollbar-track {
+  background: var(--bg-secondary);
+  border-radius: 4px;
+}
+
+.bytecode-panel::-webkit-scrollbar-thumb {
+  background: var(--border);
+  border-radius: 4px;
+}
+
+.bytecode-panel::-webkit-scrollbar-thumb:hover {
+  background: var(--text-muted);
 }
 
 /* Specialization summary bar */

Lib/profiling/sampling/_heatmap_assets/heatmap.js

@@ -542,20 +542,23 @@ function toggleBytecode(button) {
     const lineId = lineDiv.id;
     const lineNum = lineId.replace('line-', '');
     const panel = document.getElementById(`bytecode-${lineNum}`);
+    const wrapper = document.getElementById(`bytecode-wrapper-${lineNum}`);
 
-    if (!panel) return;
+    if (!panel || !wrapper) return;
 
-    const isExpanded = panel.style.display !== 'none';
+    const isExpanded = panel.classList.contains('expanded');
 
     if (isExpanded) {
-        panel.style.display = 'none';
+        panel.classList.remove('expanded');
+        wrapper.classList.remove('expanded');
         button.classList.remove('expanded');
         button.innerHTML = '▶';  // Right arrow
     } else {
         if (!panel.dataset.populated) {
             populateBytecodePanel(panel, button);
         }
-        panel.style.display = 'block';
+        panel.classList.add('expanded');
+        wrapper.classList.add('expanded');
         button.classList.add('expanded');
         button.innerHTML = '▼';  // Down arrow
     }

Lib/profiling/sampling/heatmap_collector.py

@@ -978,7 +978,17 @@ def _build_line_html(self, line_num: int, line_content: str,
                 f'data-spec-pct="{spec_pct}" '
                 f'onclick="toggleBytecode(this)" title="Show bytecode">&#9654;</button>'
             )
-            bytecode_panel_html = f'        <div class="bytecode-panel" id="bytecode-{line_num}" style="display:none;"></div>\n'
+            # Wrapper contains columns + content panel
+            bytecode_panel_html = (
+                f'        <div class="bytecode-wrapper" id="bytecode-wrapper-{line_num}">\n'
+                f'            <div class="bytecode-columns">'
+                f'<div class="line-number"></div>'
+                f'<div class="line-samples-self"></div>'
+                f'<div class="line-samples-cumulative"></div>'
+                f'</div>\n'
+                f'            <div class="bytecode-panel" id="bytecode-{line_num}"></div>\n'
+                f'        </div>\n'
+            )
         elif self.opcodes_enabled:
             # Add invisible spacer to maintain consistent indentation when opcodes are enabled
             bytecode_btn_html = '<div class="bytecode-spacer"></div>'