Switch back to clearing MathJax preview on blur

This means two clicks are required again, but that's the lesser of two
evils:

https://forums.ankiweb.net/t/anki-2-1-55-beta-mathjax-preview/23529/8

Better fixes welcome :-)

ts/editor/mathjax-overlay/MathjaxEditor.svelte

@@ -99,7 +99,7 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
         {configuration}
         bind:api={codeMirror}
         on:change={({ detail: mathjaxText }) => code.set(mathjaxText)}
-        on:tab
+        on:blur
     />
 </div>
 

ts/editor/mathjax-overlay/MathjaxOverlay.svelte

@@ -192,10 +192,7 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
                             placeHandle(true);
                             await resetHandle();
                         }}
-                        on:tab={async () => {
+                        on:blur={async () => {
-                            // Instead of resetting on blur, we reset on tab
-                            // Otherwise, when clicking from Mathjax element to another,
-                            // the user has to click twice (focus is called before blur?)
                             await resetHandle();
                         }}
                         let:editor={mathjaxEditor}