Simplify page-up/page-down

We simply move the cursor up or down by the number of whole lines that
fit on screen, allowing autoscroll to adjust the scroll position. This
causes there to be a margin under the location to which we move the
cursor, but I think it’s better to provide context and keep the
autoscroll experience consistent when using the keyboard.

spec/text-editor-spec.coffee

@@ -4535,26 +4535,21 @@ describe "TextEditor", ->
       expect(editor.getScrollBottom()).toBe (9 + editor.getVerticalScrollMargin()) * 10
 
   describe ".pageUp/Down()", ->
-    it "scrolls one screen height up or down and moves the cursor one page length", ->
+    it "moves the cursor down one page length", ->
       editor.setLineHeightInPixels(10)
       editor.setHeight(50)
-      expect(editor.getScrollHeight()).toBe 130
       expect(editor.getCursorBufferPosition().row).toBe 0
 
       editor.pageDown()
-      expect(editor.getScrollTop()).toBe 50
       expect(editor.getCursorBufferPosition().row).toBe 5
 
       editor.pageDown()
-      expect(editor.getScrollTop()).toBe 80
       expect(editor.getCursorBufferPosition().row).toBe 10
 
       editor.pageUp()
-      expect(editor.getScrollTop()).toBe 30
       expect(editor.getCursorBufferPosition().row).toBe 5
 
       editor.pageUp()
-      expect(editor.getScrollTop()).toBe 0
       expect(editor.getCursorBufferPosition().row).toBe 0
 
   describe ".selectPageUp/Down()", ->
@@ -4565,28 +4560,22 @@ describe "TextEditor", ->
       expect(editor.getCursorBufferPosition().row).toBe 0
 
       editor.selectPageDown()
-      expect(editor.getScrollTop()).toBe 30
       expect(editor.getSelectedBufferRanges()).toEqual [[[0, 0], [5, 0]]]
 
       editor.selectPageDown()
-      expect(editor.getScrollTop()).toBe 80
       expect(editor.getSelectedBufferRanges()).toEqual [[[0, 0], [10, 0]]]
 
       editor.selectPageDown()
-      expect(editor.getScrollTop()).toBe 80
       expect(editor.getSelectedBufferRanges()).toEqual [[[0, 0], [12, 2]]]
 
       editor.moveToBottom()
       editor.selectPageUp()
-      expect(editor.getScrollTop()).toBe 50
       expect(editor.getSelectedBufferRanges()).toEqual [[[7, 0], [12, 2]]]
 
       editor.selectPageUp()
-      expect(editor.getScrollTop()).toBe 0
       expect(editor.getSelectedBufferRanges()).toEqual [[[2, 0], [12, 2]]]
 
       editor.selectPageUp()
-      expect(editor.getScrollTop()).toBe 0
       expect(editor.getSelectedBufferRanges()).toEqual [[[0, 0], [12, 2]]]
 
   describe '.get/setPlaceholderText()', ->

src/text-editor.coffee

@@ -2898,14 +2898,10 @@ class TextEditor extends Model
   setVerticalScrollbarWidth: (width) -> @displayBuffer.setVerticalScrollbarWidth(width)
 
   pageUp: ->
-    newScrollTop = @getScrollTop() - @getHeight()
     @moveUp(@getRowsPerPage())
-    @setScrollTop(newScrollTop)
 
   pageDown: ->
-    newScrollTop = @getScrollTop() + @getHeight()
     @moveDown(@getRowsPerPage())
-    @setScrollTop(newScrollTop)
 
   selectPageUp: ->
     @selectUp(@getRowsPerPage())
@@ -2915,7 +2911,7 @@ class TextEditor extends Model
 
   # Returns the number of rows per page
   getRowsPerPage: ->
-    Math.max(1, Math.ceil(@getHeight() / @getLineHeightInPixels()))
+    Math.max(1, Math.floor(@getHeight() / @getLineHeightInPixels()))
 
   ###
   Section: Config