Add width: 50% to constrain the .pane width

(closes #1567)

Also remove a few `position: relative` where not needed.
Intent is to limit `position: relative` or `absolute` to only around places
that we set `left`,`top`,`right`,`bottom` properties, to avoid surprises.
(Better practice is to rely on flex for positioning instead.)

css/80_app.css

@@ -858,8 +858,6 @@ a.hide-toggle {
   display: flex;
   flex: 1 0 0px;
   flex-flow: column nowrap;
-
-  position: relative;
   overflow: hidden;
 }
 
@@ -871,23 +869,20 @@ a.hide-toggle {
   display: flex;
   flex: 1 0 0px;
   flex-flow: row nowrap;
-
-  position: relative;
   width: 200%;
 }
 
 .pane {
   display: flex;
   flex: 0 0 50%;
   flex-flow: column nowrap;
+  width: 50%;
 }
 
 .inspector-body {
   display: flex;
   flex: 1 1 0px;
   flex-flow: column nowrap;
-
-  position: relative;
   overflow-y: scroll;
   overflow-x: hidden;
 }