Demo test button for common ligatures

Part of #5231

demo/client.ts

@@ -240,6 +240,7 @@ if (document.location.pathname === '/test') {
   document.getElementById('add-decoration').addEventListener('click', addDecoration);
   document.getElementById('add-overview-ruler').addEventListener('click', addOverviewRuler);
   document.getElementById('decoration-stress-test').addEventListener('click', decorationStressTest);
+  document.getElementById('ligatures-test').addEventListener('click', ligaturesTest);
   document.getElementById('weblinks-test').addEventListener('click', testWeblinks);
   document.getElementById('bce').addEventListener('click', coloredErase);
   addVtButtons();
@@ -1307,6 +1308,18 @@ function addVtButtons(): void {
   document.querySelector('#vt-container').appendChild(vtFragment);
 }
 
+function ligaturesTest(): void {
+  term.write([
+    '',
+    '-<< -< -<- <-- <--- <<- <- -> ->> --> ---> ->- >- >>-',
+    '=<< =< =<= <== <=== <<= <= => =>> ==> ===> =>= >= >>=',
+    '<-> <--> <---> <----> <=> <==> <===> <====> :: ::: __',
+    '<~~ </ </> /> ~~> == != /= ~= <> === !== !=== =/= =!=',
+    '<: := *= *+ <* <*> *> <| <|> |> <. <.> .> +* =* =: :>',
+    '(* *) /* */ [| |] {| |} ++ +++ \/ /\ |- -| <!-- <!---'
+  ].join('\r\n'));
+}
+
 function testWeblinks(): void {
   const linkExamples = `
 aaa http://example.com aaa http://example.com aaa

demo/index.html

@@ -106,6 +106,9 @@
               <dd><button id="add-overview-ruler" title="Add an overview ruler to the terminal">Add Overview Ruler</button></dd>
               <dd><button id="decoration-stress-test" title="Toggle between adding and removing a decoration to each line">Stress Test</button></dd>
 
+              <dt>Ligatures Addon</dt>
+              <dd><button id="ligatures-test" title="Write common ligatures sequences">Common ligatures</button></dd>
+
               <dt>Weblinks Addon</dt>
               <dd><button id="weblinks-test" title="Various url conditions from demo data, hover&click to test">Test URLs</button></dd>