🐛 Fix missing forced update when minimap become visible again

If decorations were removed (as with git-diff after committing changes)
for a hidden minimap, displaying the minimap again will still display
previously rendered decorations as no update is required on display
changes.

lib/minimap-element.coffee

@@ -297,7 +297,11 @@ class MinimapElement extends HTMLElement
     @requestForcedUpdate() if @attached
 
   pollDOM: ->
-    @measureHeightAndWidth(false) if @isVisible()
+    if @isVisible()
+      @requestForcedUpdate() unless !@wasVisible
+
+      @measureHeightAndWidth(false)
+
 
   checkForVisibilityChange: ->
     if @isVisible()

spec/minimap-element-spec.coffee

@@ -258,6 +258,15 @@ describe 'MinimapElement', ->
           expect(minimapElement.canvas.width).toEqual(canvasWidth)
           expect(minimapElement.canvas.height).toEqual(canvasHeight)
 
+        describe 'from hidden to visible', ->
+          beforeEach ->
+            spyOn(minimapElement, 'requestForcedUpdate')
+            editorElement.style.display = ''
+            minimapElement.pollDOM()
+
+          it 'requests an update of the whole minimap', ->
+            expect(minimapElement.requestForcedUpdate).toHaveBeenCalled()
+
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