fix css bugs

content of rendered preview should be selectable
the hit box of width slider should has the highest z-index

browser/components/markdown.styl

@@ -54,6 +54,7 @@ body
   font-family helvetica, arial, sans-serif
   line-height 1.6
   overflow-x hidden
+  user-select all
   .katex
     font 400 1.2em 'KaTeX_Main'
     line-height 1.2em

browser/main/Main.styl

@@ -17,7 +17,6 @@
   border-width 0
   border-style solid
   border-color $ui-borderColor
-  cursor col-resize
 
 .slider--active
   @extend .slider
@@ -27,3 +26,5 @@
   absolute top bottom left right
   width 7px
   left -3px
+  z-index 10
+  cursor col-resize