don't focus textarea on tapstart for static math

we only need focus when something is selected, and the fact that
we were focusing prematurely resulted in some small behavior inconsistencies
between the first and second time e.g. that you'd click on a static table
cell in the calculator

src/services/mouse.ts

@@ -115,7 +115,8 @@ class Controller_mouse extends Controller_latex {
     };
 
     if (ctrlr.blurred) {
-      textarea.focus();
+      //for static mathquills, we focus on mousemove
+      if (this.editable) textarea.focus();
       // focus call may bubble to clients, who may then write to
       // mathquill, triggering cancelSelectionOnEdit. If that happens, we
       // don't want to stop the cursor blink or bind listeners,