compose: Handle hotkey triggers depending on whitespace in composebox.

The `focus_in_empty_compose` function used for hotkeys, now checks if
the compose box is truly empty by considering it's untrimmed value. If
there are just spaces in the focused compose box, `focus_in_empty_compose`
returns false now.

This fixes the bug where using the left key among just spaces did not move
back the cursor as expected, and may unexpectedly trigger edit state for
the last sent message.

For `up` hotkeys, message navigation is also triggered if the cursor is
at the start of the composebox with just whitespace. A new helper
function is added for this check.

static/js/compose_state.js

@@ -31,7 +31,7 @@ export function composing() {
     return Boolean(message_type);
 }
 
-function get_or_set(fieldname, keep_leading_whitespace) {
+function get_or_set(fieldname, keep_leading_whitespace, no_trim) {
     // We can't hoist the assignment of '$elem' out of this lambda,
     // because the DOM element might not exist yet when get_or_set
     // is called.
@@ -41,7 +41,12 @@ function get_or_set(fieldname, keep_leading_whitespace) {
         if (newval !== undefined) {
             $elem.val(newval);
         }
-        return keep_leading_whitespace ? oldval.trimEnd() : oldval.trim();
+        if (no_trim) {
+            return oldval;
+        } else if (keep_leading_whitespace) {
+            return oldval.trimEnd();
+        }
+        return oldval.trim();
     };
 }
 
@@ -63,11 +68,18 @@ export const topic = get_or_set("stream_message_recipient_topic");
 // of the indented syntax for multi-line code blocks.
 export const message_content = get_or_set("compose-textarea", true);
 
+const untrimmed_message_content = get_or_set("compose-textarea", true, true);
+
+export function cursor_at_start_of_whitespace_in_compose() {
+    const cursor_position = $("#compose-textarea").caret();
+    return message_content() === "" && cursor_position === 0;
+}
+
 export function focus_in_empty_compose() {
     // A user trying to press arrow keys in an empty compose is mostly
     // likely trying to navigate messages. This helper function
-    // decides whether the compose box is "empty" for this purpose.
-    if (!composing() || message_content() !== "") {
+    // decides whether the compose box is empty for this purpose.
+    if (!composing() || untrimmed_message_content() !== "") {
         return false;
     }
 

static/js/hotkey.js

@@ -735,13 +735,11 @@ export function process_hotkey(e, hotkey) {
         }
 
         if (
-            (event_name === "up_arrow" ||
-                event_name === "down_arrow" ||
-                event_name === "page_up" ||
-                event_name === "page_down" ||
-                event_name === "home" ||
-                event_name === "end") &&
-            compose_state.focus_in_empty_compose()
+            ((event_name === "down_arrow" || event_name === "page_down" || event_name === "end") &&
+                compose_state.focus_in_empty_compose()) ||
+            ((event_name === "up_arrow" || event_name === "page_up" || event_name === "home") &&
+                (compose_state.focus_in_empty_compose() ||
+                    compose_state.cursor_at_start_of_whitespace_in_compose()))
         ) {
             compose_actions.cancel();
             // don't return, as we still want it to be picked up by the code below