hgweb: add a floating tooltip to invite on followlines action

In followlines.js, we create a <div id="followlines-tooltip"> element to draw
attention of users on "followlines" feature. The element shows up on hover of
source lines after one second and follows the cursor. After first click (start
line selection), the text changes and indicates that next click will terminate
selection.

mercurial/templates/static/followlines.js

@@ -17,6 +17,34 b" document.addEventListener('DOMContentLoa"
         return;
     }
 
+    // tooltip to invite on lines selection
+    var tooltip = document.createElement('div');
+    tooltip.id = 'followlines-tooltip';
+    tooltip.classList.add('hidden');
+    var initTooltipText = 'click to start following lines history from here';
+    tooltip.textContent = initTooltipText;
+    sourcelines.appendChild(tooltip);
+
+    var tooltipTimeoutID;
+    //* move the "tooltip" with cursor (top-right) and show it after 1s */
+    function moveAndShowTooltip(e) {
+        if (typeof tooltipTimeoutID !== 'undefined') {
+            // avoid accumulation of timeout callbacks (blinking)
+            window.clearTimeout(tooltipTimeoutID);
+        }
+        tooltip.classList.add('hidden');
+        var x = (e.clientX + 10) + 'px',
+            y = (e.clientY - 20) + 'px';
+        tooltip.style.top = y;
+        tooltip.style.left = x;
+        tooltipTimeoutID = window.setTimeout(function() {
+            tooltip.classList.remove('hidden');
+        }, 1000);
+    }
+
+    // on mousemove, show tooltip close to cursor position
+    sourcelines.addEventListener('mousemove', moveAndShowTooltip);
+
     // retrieve all direct <span> children of <pre class="sourcelines">
     var spans = Array.prototype.filter.call(
         sourcelines.children,
@@ -65,6 +93,10 b" document.addEventListener('DOMContentLoa"
             // registered for other click with <span> target
             return;
         }
+
+        // update tooltip text
+        tooltip.textContent = 'click again to terminate line block selection here';
+
         var startId = parseInt(startElement.id.slice(1));
         startElement.classList.add(lineSelectedCSSClass); // CSS
 
@@ -83,13 +115,25 b" document.addEventListener('DOMContentLoa"
             // remove this event listener
             sourcelines.removeEventListener('click', lineSelectEnd);
 
+            // hide tooltip and disable motion tracking
+            tooltip.classList.add('hidden');
+            sourcelines.removeEventListener('mousemove', moveAndShowTooltip);
+            window.clearTimeout(tooltipTimeoutID);
+
+            //* restore initial "tooltip" state */
+            function restoreTooltip() {
+                tooltip.textContent = initTooltipText;
+                sourcelines.addEventListener('mousemove', moveAndShowTooltip);
+            }
+
             // compute line range (startId, endId)
             var endId = parseInt(endElement.id.slice(1));
             if (endId == startId) {
                 // clicked twice the same line, cancel and reset initial state
-                // (CSS and event listener for selection start)
+                // (CSS, event listener for selection start, tooltip)
                 removeSelectedCSSClass();
                 sourcelines.addEventListener('click', lineSelectStart);
+                restoreTooltip();
                 return;
             }
             var inviteElement = endElement;
@@ -118,6 +162,8 b" document.addEventListener('DOMContentLoa"
                 sourcelines.removeEventListener('click', cancel);
                 // remove styles on selected lines
                 removeSelectedCSSClass();
+                // restore tooltip element
+                restoreTooltip();
             }
 
             // bind cancel event to click on <button>

mercurial/templates/static/style-paper.css

@@ -320,6 +320,22 b' div.followlines-link {'
   font-family: sans-serif;
 }
 
+div#followlines-tooltip {
+  display: none;
+  position: fixed;
+  background-color: #ffc;
+  border: 1px solid #999;
+  padding: 2px;
+}
+
+.sourcelines:hover > div#followlines-tooltip {
+  display: inline;
+}
+
+.sourcelines:hover > div#followlines-tooltip.hidden {
+  display: none;
+}
+
 .sourcelines > a {
     display: inline-block;
     position: absolute;