Bangle.js: Fix back handler not removed when using E.setUI with a back button but without widgets (#2636)

ChangeLog

@@ -1,4 +1,5 @@
           : Add `E.internal` as a way to access the 'hidden root' containing Espruino internal variables that previously needed `global["\xff"]`
+            Bangle.js: Fix back handler not removed when using E.setUI with a back button but without widgets (#2636)
 
      2v27 : nRF5x: Ensure Bluetooth notifications work correctly when two separate connections use the same handle for their characteristics
             nRF5x: Remove handlers from our handlers array when a device is disconnected

libs/js/banglejs/Bangle_setUI_F18.js

@@ -92,13 +92,12 @@
   if (options.redraw) // handler for redrawing the UI
     Bangle.uiRedraw = options.redraw;
   if (options.back) {
-    var btnWatch;
     // only add back button handler if there's no existing watch on BTN1
     if (Bangle.btnWatches===undefined) 
-      btnWatch = setWatch(function() {
-        btnWatch = undefined;
+      Bangle.btnWatches = [ setWatch(function() {
+        Bangle.btnWatches = undefined; // watch doesn't repeat
         options.back();
-      }, BTN3, {edge:"rising"});
+      }, BTN3, {edge:"rising"}) ];
     // if we have widgets loaded *and* visible at the top, add a back widget (see #3788)
     if (global.WIDGETS && Bangle.appRect.y) {
       // add our own touch handler for touching in the left
@@ -116,7 +115,6 @@
         remove:function(noclear){
           var w = WIDGETS.back;
           if (w.area!="tl") noclear=true; // area="" is set by widget_utils.hide, so avoid drawing
-          if (btnWatch) clearWatch(btnWatch);
           Bangle.removeListener("touch", touchHandler);
           if (!noclear) g.reset().clearRect({x:w.x, y:w.y, w:24,h:24});
           delete WIDGETS.back;

libs/js/banglejs/Bangle_setUI_Q3.js

@@ -119,12 +119,11 @@
     Bangle.uiRedraw = options.redraw;
   if (options.back) {
     // only add back button handler if there's no existing watch on BTN1
-    if (Bangle.btnWatches===undefined) {
+    if (Bangle.btnWatches===undefined)
       Bangle.btnWatches = [ setWatch(function() {
         Bangle.btnWatches = undefined; // watch doesn't repeat
         options.back();
       }, BTN1, {edge:"rising"}) ];
-    }
     // if we have widgets loaded *and* visible at the top, add a back widget (see #3788)
     if (global.WIDGETS && Bangle.appRect.y) {
       // add our own touch handler for touching in the top left