VueRouter - Allow scrolling other than window

active-rfcs/0000-vue-router-scrolling-element.md

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+- Start Date: 2019-05-22
+- Target Major Version: Both?
+- Reference Issues: https://github.com/vuejs/vue-router/issues/1187
+- Implementation PR: https://github.com/vuejs/vue-router/pull/2780 (already did the PR so...) 
+
+# Summary
+
+### Vue Router - Scroll Behavior 
+
+In a lot of cases, the scrolling element in an app is not the window, 
+this RFC attempts to allow the user to specify the scrolling element in their app.
+
+# Basic example
+
+This should be pretty straight forward, in the router options:
+
+```javascript
+new Router({
+   //...
+   scrollElement: '.scroller' //string or Dom Reference
+})
+```
+
+# Motivation
+
+In order to allow for more cases of scrolling, and saving positions of scrolling 
+and scrolling with scrollBehavior when the window is not the scroller, this was requested
+a while ago already in https://github.com/vuejs/vue-router/issues/1187. 
+
+This shouldn't incur any breaking changes, only adding an extra option, so should be seamless to the users.
+
+# Detailed design
+
+Element Selector is a new property on the router instance (options),
+and can also be passed by scrollBehavior results (same as selector is passed) to override the global one.
+
+The scrolling itself is pretty simple - you get the scrolling element by passing the selector/dom element/null:
+
+```javascript
+function getScrollElement (scrollElementSelector?: any): Element | WindowProxy {
+  let domScrollElement = window
+  if (!scrollElementSelector) {
+    return window
+  }
+
+  assert(typeof scrollElementSelector === 'string' || scrollElementSelector instanceof Element, 'Scroll Element must be a css selector string or a DOM element')
+  if (typeof scrollElementSelector === 'string') {
+    const customScrollElement = document.querySelector(scrollElementSelector)
+    if (customScrollElement) {
+      domScrollElement = customScrollElement
+    }
+  } else if (scrollElementSelector instanceof Element) {
+    return scrollElementSelector
+  }
+
+  return domScrollElement
+}
+```
+
+Using this element you can save, and / or scroll.
+
+
+# Drawbacks
+
+- Adds a bit of complexity to saving and setting scroll - need to access different properties (scrollTo  / scrollLeft / pageXOffset etc) because of old browsers quirks.
+- Need to pass more info between functions that were purer before (such as saveScrollPosition that now needs to get an optional scroll selector)
+- If scroller changes this might cause some unexpected behaviors.
+
+
+Why should we *not* do this? Please consider:
+
+- Slight increase in complexity
+- New slightly complicated tests scenarios.
+- Will need to write twice (v2 and v3)
+
+# Alternatives
+
+Another way that was discussed in the the original issue is to allow the user to define 2 new functions - saveScrollPosition and scrollTo
+which the router will use instead of the default ones.
+
+Doing it this way (element selector) will solve for most of the cases in a simpler setup.
+
+# Adoption strategy
+
+Incremental Adoption - users don't need to actively change anything if upgrading.
+
+# Unresolved questions
+
+The save scroll position required sending the scrolling element from a lot of different places - perhaps there is a better way to do this.
+If the scroll element changes it may cause unexpected behavior with saved positions - but I am not sure it's worth worrying about - since it's only on "pop" states where it shouldn't really change.