Prevent scrollLeft/scrollTop from going out of bounds

spec/display-buffer-spec.coffee

@@ -955,3 +955,41 @@ describe "DisplayBuffer", ->
         {start, end} = marker.getPixelRange()
         expect(start.top).toBe 5 * 20
         expect(start.left).toBe (4 * 10) + (6 * 11)
+
+  describe "::setScrollTop", ->
+    beforeEach ->
+      displayBuffer.setLineHeight(10)
+
+    it "disallows negative values", ->
+      displayBuffer.setHeight(displayBuffer.getScrollHeight() + 100)
+      expect(displayBuffer.setScrollTop(-10)).toBe 0
+      expect(displayBuffer.getScrollTop()).toBe 0
+
+    it "disallows values that would make ::getScrollBottom() exceed ::getScrollHeight()", ->
+      displayBuffer.setHeight(50)
+      maxScrollTop = displayBuffer.getScrollHeight() - displayBuffer.getHeight()
+
+      expect(displayBuffer.setScrollTop(maxScrollTop)).toBe maxScrollTop
+      expect(displayBuffer.getScrollTop()).toBe maxScrollTop
+
+      expect(displayBuffer.setScrollTop(maxScrollTop + 50)).toBe maxScrollTop
+      expect(displayBuffer.getScrollTop()).toBe maxScrollTop
+
+  describe "::setScrollLeft", ->
+    beforeEach ->
+      displayBuffer.setDefaultCharWidth(10)
+
+    it "disallows negative values", ->
+      displayBuffer.setWidth(displayBuffer.getScrollWidth() + 100)
+      expect(displayBuffer.setScrollLeft(-10)).toBe 0
+      expect(displayBuffer.getScrollLeft()).toBe 0
+
+    it "disallows values that would make ::getScrollRight() exceed ::getScrollWidth()", ->
+      displayBuffer.setWidth(50)
+      maxScrollLeft = displayBuffer.getScrollWidth() - displayBuffer.getWidth()
+
+      expect(displayBuffer.setScrollLeft(maxScrollLeft)).toBe maxScrollLeft
+      expect(displayBuffer.getScrollLeft()).toBe maxScrollLeft
+
+      expect(displayBuffer.setScrollLeft(maxScrollLeft + 50)).toBe maxScrollLeft
+      expect(displayBuffer.getScrollLeft()).toBe maxScrollLeft

src/display-buffer.coffee

@@ -113,7 +113,7 @@ class DisplayBuffer extends Model
 
   getScrollTop: -> @scrollTop
   setScrollTop: (scrollTop) ->
-    @scrollTop = Math.min(@getScrollHeight() - @getHeight(), Math.max(0, scrollTop))
+    @scrollTop = Math.max(0, Math.min(@getScrollHeight() - @getHeight(), scrollTop))
 
   getScrollBottom: -> @scrollTop + @height
   setScrollBottom: (scrollBottom) ->
@@ -122,7 +122,7 @@ class DisplayBuffer extends Model
 
   getScrollLeft: -> @scrollLeft
   setScrollLeft: (scrollLeft) ->
-    @scrollLeft = Math.min(@getScrollWidth() - @getWidth(), Math.max(0, scrollLeft))
+    @scrollLeft = Math.max(0, Math.min(@getScrollWidth() - @getWidth(), scrollLeft))
 
   getScrollRight: -> @scrollLeft + @width
   setScrollRight: (scrollRight) ->