🐎 Cache screen row height

I am trying to defer the usage of fancy algorithms as much as possible. The
current one is linear and should probably be changed, but it performs quite
decently for the time being. Maybe with some more caching we could even avoid to
implement a tree data structure?

src/text-editor-presenter.coffee

@@ -30,6 +30,7 @@ class TextEditorPresenter
     @customGutterDecorationsByGutterName = {}
     @blockDecorationsDimensionsById = {}
     @blockDecorationsDimensionsByScreenRow = {}
+    @heightsByScreenRow = {}
     @screenRowsToMeasure = []
     @transferMeasurementsToModel()
     @transferMeasurementsFromModel()
@@ -426,17 +427,20 @@ class TextEditorPresenter
 
       continue if rowsWithinTile.length is 0
 
+      top = @positionForRow(tileStartRow)
+      height = @positionForRow(tileStartRow + @tileSize) - top
+
       tile = @state.content.tiles[tileStartRow] ?= {}
-      tile.top = @positionForRow(tileStartRow) - @scrollTop
+      tile.top = top - @scrollTop
       tile.left = -@scrollLeft
-      tile.height = @positionForRow(tileStartRow + @tileSize) - @positionForRow(tileStartRow)
+      tile.height = height
       tile.display = "block"
       tile.zIndex = zIndex
       tile.highlights ?= {}
 
       gutterTile = @lineNumberGutter.tiles[tileStartRow] ?= {}
-      gutterTile.top = @positionForRow(tileStartRow) - @scrollTop
-      gutterTile.height = @positionForRow(tileStartRow + @tileSize) - @positionForRow(tileStartRow)
+      gutterTile.top = top - @scrollTop
+      gutterTile.height = height
       gutterTile.display = "block"
       gutterTile.zIndex = zIndex
 
@@ -693,12 +697,17 @@ class TextEditorPresenter
 
     return
 
+  getScreenRowHeight: (screenRow) ->
+    @heightsByScreenRow[screenRow] or @lineHeight
+
+  setScreenRowHeight: (screenRow, height) ->
+    @heightsByScreenRow[screenRow] = height
+
   rowForPosition: (position, floor = true) ->
     top = 0
     for tileRow in [0..@model.getScreenLineCount()] by @tileSize
       for row in [tileRow...Math.min(tileRow + @tileSize, @model.getScreenLineCount())] by 1
-        blockDecorationsForCurrentRow = _.values(@blockDecorationsDimensionsByScreenRow[row])
-        nextTop = top + @lineHeight + @getBlockDecorationsHeight(blockDecorationsForCurrentRow)
+        nextTop = top + @getScreenRowHeight(row)
         if floor
           return row if nextTop > position
         else
@@ -711,8 +720,7 @@ class TextEditorPresenter
     for tileRow in [0..@model.getScreenLineCount()] by @tileSize
       for row in [tileRow...Math.min(tileRow + @tileSize, @model.getScreenLineCount())] by 1
         return top if row is targetRow
-        blockDecorationsForNextRow = _.values(@blockDecorationsDimensionsByScreenRow[row + 1])
-        top += @lineHeight + @getBlockDecorationsHeight(blockDecorationsForNextRow)
+        top += @getScreenRowHeight(row + 1)
     top
 
   updateStartRow: ->
@@ -1403,10 +1411,15 @@ class TextEditorPresenter
     screenRow = decoration.getMarker().getHeadScreenPosition().row
     dimensions = {width, height}
 
-    @blockDecorationsDimensionsByScreenRow[screenRow] ?= {}
-    @blockDecorationsDimensionsByScreenRow[screenRow][decoration.id] = dimensions
+    screenRowDecorations = @blockDecorationsDimensionsByScreenRow[screenRow] ?= {}
+    screenRowDecorations[decoration.id] = dimensions
     @blockDecorationsDimensionsById[decoration.id] = dimensions
 
+    @setScreenRowHeight(
+      screenRow,
+      @lineHeight + @getBlockDecorationsHeight(_.values(screenRowDecorations))
+    )
+
     @shouldUpdateBlockDecorations = true
     @shouldUpdateVerticalScrollState = true
     @emitDidUpdateState()