TextEditor: Fix annotation rectangle caching

The rectangle of an annotation is cached after it was painted. If just a
part of an editor was updated all annotations were removed but only
redrawn annotations were added to the cache again.

This behavior is replaced by removing annotations that are not visible
and those which got redrawn. So annotations that are still visible but
were not redrawn are kept in the cache.

Change-Id: I9246f1347b8f795284fb4fc9aabe11f251d16c25
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/texteditor/texteditor.cpp

@@ -4051,7 +4051,6 @@ void TextEditorWidget::paintEvent(QPaintEvent *e)
     int cursor_cpos = 0;
     QPen cursor_pen;
 
-    d->m_annotationRects.clear();
     d->m_searchResultOverlay->clear();
     if (!d->m_searchExpr.isEmpty()) { // first pass for the search result overlays
 
@@ -4206,6 +4205,12 @@ void TextEditorWidget::paintEvent(QPaintEvent *e)
                                    fs.toTextCharFormat(C_SEARCH_RESULT).background().color(),
                                    e->rect());
 
+    { // remove all annotation rects from the cache that where drawn before the first visible block
+        auto it = d->m_annotationRects.begin();
+        auto end = d->m_annotationRects.end();
+        while (it != end && it.key() < block.blockNumber())
+            it = d->m_annotationRects.erase(it);
+    }
 
     while (block.isValid()) {
 
@@ -4421,6 +4426,7 @@ void TextEditorWidget::paintEvent(QPaintEvent *e)
                     && blockSelectionCursorRect.isValid())
                 painter.fillRect(blockSelectionCursorRect, palette().text());
 
+            d->m_annotationRects.remove(block.blockNumber());
             d->drawLineAnnotation(painter, block, lineX < viewportRect.width() ? lineX : 0, er);
         }
 
@@ -4441,6 +4447,19 @@ void TextEditorWidget::paintEvent(QPaintEvent *e)
             block = doc->findBlockByLineNumber(block.firstLineNumber());
         }
     }
+
+    // remove all annotation rects from the cache that where drawn after the last visible block
+    if (block.isValid()) {
+        auto it = d->m_annotationRects.begin();
+        auto end = d->m_annotationRects.end();
+        while (it != end) {
+            if (it.key() > block.blockNumber())
+                it = d->m_annotationRects.erase(it);
+            else
+                ++it;
+        }
+    }
+
     painter.setPen(context.palette.text().color());
 
     if (backgroundVisible() && !block.isValid() && offset.y() <= er.bottom()