Correctly remove block decorations whose markers have been destroyed

In https://github.com/atom/atom/pull/15503 we mistakenly assumed
`marker.isValid` accounted only for the validity of the marker. However,
that method returns `false` also for markers that are valid but have
been destroyed. As a result, the editor component was mistakenly not
removing block decorations associated with such markers.

With this commit we will rely on the local `wasValid` variable instead.
If its value is `true`, it means that the block decoration has been
accounted for in the `lineTopIndex` and must, as a result, be cleaned up
in case the marker or the decoration gets destroyed.

spec/text-editor-component-spec.js

@@ -2299,6 +2299,27 @@ describe('TextEditorComponent', () => {
       ])
     })
 
+    it('removes block decorations whose markers have been destroyed', async () => {
+      const {editor, component, element} = buildComponent({rowsPerTile: 3})
+      const {marker} = createBlockDecorationAtScreenRow(editor, 2, {height: 5, position: 'before'})
+      await component.getNextUpdatePromise()
+      assertLinesAreAlignedWithLineNumbers(component)
+      assertTilesAreSizedAndPositionedCorrectly(component, [
+        {tileStartRow: 0, height: 3 * component.getLineHeight() + 5},
+        {tileStartRow: 3, height: 3 * component.getLineHeight()},
+        {tileStartRow: 6, height: 3 * component.getLineHeight()}
+      ])
+
+      marker.destroy()
+      await component.getNextUpdatePromise()
+      assertLinesAreAlignedWithLineNumbers(component)
+      assertTilesAreSizedAndPositionedCorrectly(component, [
+        {tileStartRow: 0, height: 3 * component.getLineHeight()},
+        {tileStartRow: 3, height: 3 * component.getLineHeight()},
+        {tileStartRow: 6, height: 3 * component.getLineHeight()}
+      ])
+    })
+
     it('removes block decorations whose markers are invalidated, and adds them back when they become valid again', async () => {
       const editor = buildEditor({rowsPerTile: 3, autoHeight: false})
       const {item, decoration, marker} = createBlockDecorationAtScreenRow(editor, 3, {height: 44, position: 'before', invalidate: 'touch'})

src/text-editor-component.js

@@ -2506,7 +2506,7 @@ class TextEditorComponent {
         didUpdateDisposable.dispose()
         didDestroyDisposable.dispose()
 
-        if (marker.isValid()) {
+        if (wasValid) {
           this.blockDecorationsToMeasure.delete(decoration)
           this.heightsByBlockDecoration.delete(decoration)
           this.blockDecorationsByElement.delete(element)