message_list: Rename confusing hide-date CSS class.

The previous hide-date CSS class had a semantic meaning of "this
recipient row has the same date as the previous one"; since we're now
having logic read that value, it's worth giving it a semantic name
that makes that code easier to understand.

web/src/message_list_view.js

@@ -116,12 +116,12 @@ function update_group_date(group, message_container, prev) {
     const today = new Date();
 
     // Show the date in the recipient bar if the previous message was from a different day.
-    group.show_recipient_bar_date = !same_day(message_container, prev);
+    group.date_unchanged = same_day(message_container, prev);
     group.group_date_html = timerender.render_date(time, today)[0].outerHTML;
 }
 
 function clear_group_date(group) {
-    group.show_recipient_bar_date = true;
+    group.date_unchanged = false;
     group.group_date_html = undefined;
 }
 
@@ -1542,12 +1542,9 @@ export class MessageListView {
         // hiding the date display on the non-sticky previous
         // recipient row.
         $(".hide-date-separator-header").removeClass("hide-date-separator-header");
-        // The hide-date CSS class is set on recipient_row_date
-        // elements where the previous row had the same date; these
-        // will only display the date when sticky. Since this corner
-        // case only is relevant with an identical date, we start by
-        // checking whether the hide-date class is present.
-        if ($sticky_header.find(".recipient_row_date.hide-date").length) {
+        // This corner case only occurs when the date is unchanged
+        // from the previous recipient row.
+        if ($sticky_header.find(".recipient_row_date.recipient_row_date_unchanged").length) {
             const $prev_recipient_row = $sticky_header
                 .closest(".recipient_row")
                 .prev(".recipient_row");
@@ -1556,7 +1553,7 @@ export class MessageListView {
             }
             const $prev_header_date_row = $prev_recipient_row.find(".recipient_row_date");
             // Check if the recipient row before sticky header is a date separator.
-            if (!$prev_header_date_row.hasClass("hide-date")) {
+            if (!$prev_header_date_row.hasClass("recipient_row_date_unchanged")) {
                 $prev_header_date_row.addClass("hide-date-separator-header");
             }
         }

web/styles/zulip.css

@@ -1100,12 +1100,6 @@ td.pointer {
         top: var(--sidebar-top);
         z-index: 3;
         box-shadow: 0 -1px 0 0 hsl(0, 0%, 100%);
-
-        &.sticky_header {
-            .recipient_row_date {
-                display: block;
-            }
-        }
     }
 }
 
@@ -1181,7 +1175,15 @@ td.pointer {
     letter-spacing: 0.04em;
     text-transform: uppercase;
 
-    &.hide-date,
+    /* Display the date when unchanged only for sticky headers. */
+    &.recipient_row_date_unchanged {
+        display: none;
+
+        .sticky_header & {
+            display: block;
+        }
+    }
+
     &.hide-date-separator-header {
         display: none;
     }

web/templates/recipient_row.hbs

@@ -77,7 +77,7 @@
                     </template>
                 {{/if}}
             </span>
-            <span class="recipient_row_date {{#if show_recipient_bar_date}}{{else}}hide-date{{/if}}">{{{date}}}</span>
+            <span class="recipient_row_date {{#if date_unchanged}}recipient_row_date_unchanged{{/if}}">{{{date}}}</span>
         </div>
     </div>
 </div>
@@ -91,7 +91,7 @@
                 {{t "You and {display_reply_to}" }}
             </a>
 
-            <span class="recipient_row_date {{#if show_recipient_bar_date}}{{else}}hide-date{{/if}}">{{{date}}}</span>
+            <span class="recipient_row_date {{#if date_unchanged}}recipient_row_date_unchanged{{/if}}">{{{date}}}</span>
         </div>
     </div>
 </div>