compose: Only transition when low-attention row hovered.

Previously, if the mouse cursor were in the vicinity of
the compose box, we'd see transitions even on keyboard
interactions, which should not have been happening.

This also allows instantaneous highlighting for keyboard
users, as well as avoiding any transitions when a
mouse user clicks into a recipient-row element (at
which point the hover transitions will have already
run).

web/styles/compose.css

@@ -1241,9 +1241,10 @@ textarea.new_message_textarea {
    the mouse, once the compose box is open. Any keyboard
    interactions (e.g., Shift-Tabbing from the compose textarea
    to the topic box) show instant changes, so we don't need to
-   accommodate them here. */
-#compose.compose-box-open:hover {
-    .low-attention-recipient-row {
+   accommodate them here, which we prevent by applying the
+   transitions only when focus isn't within the recipient row. */
+#compose.compose-box-open {
+    .low-attention-recipient-row:hover:not(:focus-within) {
         #compose_select_recipient_widget,
         #compose_recipient_box,
         #compose-direct-recipient .pill-container {