From 1e56f7763e85ff6f0178fc12a0f9a01c56b41503 Mon Sep 17 00:00:00 2001
From: spacejack <spacejack@gmail.com>
Date: Sun, 13 Aug 2017 18:32:32 -0400
Subject: [PATCH 1/2] More accurate style object diffs

---
 render/render.js                   | 12 ++++++++++++
 render/tests/test-updateElement.js | 13 +++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/render/render.js b/render/render.js
index 6bebd212..40eca443 100644
--- a/render/render.js
+++ b/render/render.js
@@ -552,6 +552,18 @@ module.exports = function($window) {
 
 	//style
 	function updateStyle(element, old, style) {
+		if (old != null && style != null && typeof old === "object" && typeof style === "object" && style !== old) {
+			// Both old & new are (different) objects.
+			// Update style properties that have changed
+			for (var key in style) {
+				if (style[key] !== old[key]) element.style[key] = style[key]
+			}
+			// Remove style properties that no longer exist
+			for (var key in old) {
+				if (!(key in style)) element.style[key] = ""
+			}
+			return
+		}
 		if (old === style) element.style.cssText = "", old = null
 		if (style == null) element.style.cssText = ""
 		else if (typeof style === "string") element.style.cssText = style
diff --git a/render/tests/test-updateElement.js b/render/tests/test-updateElement.js
index 313fe1a9..a2ae0fcb 100644
--- a/render/tests/test-updateElement.js
+++ b/render/tests/test-updateElement.js
@@ -192,6 +192,19 @@ o.spec("updateElement", function() {
 		o(updated.dom.style.backgroundColor).equals("")
 		o(updated.dom.style.color).equals("gold")
 	})
+	o("does not re-render element styles for equivalent style objects", function() {
+		var style = {color: "gold"}
+		var vnode = {tag: "a", attrs: {style: style}}
+
+		render(root, [vnode])
+
+		root.firstChild.style.color = "red"
+		style = {color: "gold"}
+		var updated = {tag: "a", attrs: {style: style}}
+		render(root, [updated])
+
+		o(updated.dom.style.color).equals("red")
+	})
 	o("replaces el", function() {
 		var vnode = {tag: "a"}
 		var updated = {tag: "b"}

From ae0b7915300881d8c5a685d8df36176416a70879 Mon Sep 17 00:00:00 2001
From: spacejack <spacejack@gmail.com>
Date: Tue, 15 Aug 2017 15:53:23 -0400
Subject: [PATCH 2/2] Eleminate now-redundant if block. Add change-log notes.

---
 docs/change-log.md | 1 +
 render/render.js   | 5 -----
 2 files changed, 1 insertion(+), 5 deletions(-)

diff --git a/docs/change-log.md b/docs/change-log.md
index 6ea8b86a..82686f0e 100644
--- a/docs/change-log.md
+++ b/docs/change-log.md
@@ -27,6 +27,7 @@
 
 - API: `m.route.set()` causes all mount points to be redrawn ([#1592](https://github.com/MithrilJS/mithril.js/pull/1592))
 - API: If a user sets the Content-Type header within a request's options, that value will be the entire header value rather than being appended to the default value ([#1924](https://github.com/MithrilJS/mithril.js/pull/1924))
+- API: Using style objects in hyperscript calls will now properly diff style properties from one render to another as opposed to re-writing all element style properties every render.
 
 ---
 
diff --git a/render/render.js b/render/render.js
index 40eca443..f6782297 100644
--- a/render/render.js
+++ b/render/render.js
@@ -572,11 +572,6 @@ module.exports = function($window) {
 			for (var key in style) {
 				element.style[key] = style[key]
 			}
-			if (old != null && typeof old !== "string") {
-				for (var key in old) {
-					if (!(key in style)) element.style[key] = ""
-				}
-			}
 		}
 	}