Don't try and avoid updating state

I'm not sure if this is the best solution for tomkp#309, as I am not entirely aware of what this code was trying to do, but it *seemed* like it was trying to avoid touching state too much which doesn't seem super useful as there wasn't any complex calculations involved -- and if there was, memoization would be more useful.

In any case it was triggering problems in strict mode which implies it would have broken in later version of React.

src/SplitPane.js

@@ -37,7 +37,7 @@ function getDefaultSize(defaultSize, minSize, maxSize, draggedSize) {
 }
 
 function removeNullChildren(children) {
-    return React.Children.toArray(children).filter(c => c);
+  return React.Children.toArray(children).filter(c => c);
 }
 class SplitPane extends React.Component {
   constructor(props) {
@@ -65,15 +65,6 @@ class SplitPane extends React.Component {
       resized: false,
       pane1Size: primary === 'first' ? initialSize : undefined,
       pane2Size: primary === 'second' ? initialSize : undefined,
-
-      // previous props that we need in static methods
-      instanceProps: {
-        primary,
-        size,
-        defaultSize,
-        minSize,
-        maxSize,
-      },
     };
   }
 
@@ -214,7 +205,6 @@ class SplitPane extends React.Component {
   // TODO: find a more elegant way to fix this. memoize calls to setSize?
   // we have to check values since gDSFP is called on every render
   static setSize(props, state) {
-    const { instanceProps } = state;
     const newState = {};
 
     const newSize =
@@ -227,43 +217,14 @@ class SplitPane extends React.Component {
             state.draggedSize
           );
 
-    const defaultSizeChanged =
-      props.defaultSize !== instanceProps.defaultSize ||
-      props.minSize !== instanceProps.minSize ||
-      props.maxSize !== instanceProps.maxSize;
-
-    const shouldUpdateSize =
-      props.size !== undefined
-        ? props.size !== instanceProps.size
-        : defaultSizeChanged;
-
-    if (
-      props.size !== undefined &&
-      props.size !== state.draggedSize &&
-      shouldUpdateSize
-    ) {
+    if (props.size !== undefined) {
       newState.draggedSize = newSize;
     }
 
     const isPanel1Primary = props.primary === 'first';
 
-    if (shouldUpdateSize || props.primary !== state.instanceProps.primary) {
-      newState[isPanel1Primary ? 'pane1Size' : 'pane2Size'] = newSize;
-    }
-
-    // unset the size on the non primary panel
-    if (props.primary !== state.instanceProps.primary) {
-      newState[isPanel1Primary ? 'pane2Size' : 'pane1Size'] = undefined;
-    }
-
-    // update the values in instanceProps
-    instanceProps.primary = props.primary;
-    instanceProps.size = props.size;
-    instanceProps.defaultSize = props.defaultSize;
-    instanceProps.minSize = props.minSize;
-    instanceProps.maxSize = props.maxSize;
-
-    newState.instanceProps = instanceProps;
+    newState[isPanel1Primary ? 'pane1Size' : 'pane2Size'] = newSize;
+    newState[isPanel1Primary ? 'pane2Size' : 'pane1Size'] = undefined;
 
     return newState;
   }
@@ -296,7 +257,7 @@ class SplitPane extends React.Component {
       : resizerClassName;
 
     const notNullChildren = removeNullChildren(children);
-    
+
     const style = Object.assign(
       {},
       {