FIX REPL tab at beginning of line should indent not complete (#2125)

docs/project/changelog.md

@@ -36,6 +36,9 @@ substitutions:
   improved with some context information.
   {pr}`2121`
 
+- {{Enhancement}} Pressing TAB in REPL no longer triggers completion when input
+  is whitespace. {pr}`2125`
+
 ## Version 0.19.0
 
 _January 10, 2021_

src/templates/console.html

@@ -133,6 +133,15 @@
               term.set_command("");
               term.set_prompt(ps1);
             },
+            "TAB": (event, original) => {
+              const command = term.before_cursor();
+              // Disable completion for whitespaces.
+              if (command.trim() === "") {
+                term.insert("\t");
+                return false;
+              }
+              return original(event);
+            }
           },
         });
         window.term = term;