tippy: Initialize maxWidth in em units.

web/src/compose_tooltips.ts

@@ -240,7 +240,8 @@ export function initialize(): void {
         // TODO: Might need to target just the Send button itself
         // in the new design
         target: ".disabled-message-send-controls",
-        maxWidth: 350,
+        // 350px at 14px/1em
+        maxWidth: "25em",
         content: () =>
             compose_recipient.get_posting_policy_error_message() ||
             compose_validate.get_disabled_send_tooltip(),

web/src/tippyjs.ts

@@ -53,7 +53,8 @@ export const EXTRA_LONG_HOVER_DELAY: [number, number] = [1500, 20];
 // documentation for default properties.
 tippy.default.setDefaultProps({
     // Tooltips shouldn't take more space than mobile widths.
-    maxWidth: 300,
+    // 300px at 14px/1em
+    maxWidth: "21.4286em",
     delay: INSTANT_HOVER_DELAY,
     placement: "top",
     // Disable animations to make the tooltips feel snappy.