scene: Pad damage if scale factor is fractional

Floating point arithmetic and/or the use of GL_LINEAR in the pipeline,
may result in visual glitches due to rounding (errors). As a short-term
fix until a proper (more involved) solution is in place for Plasma 6,
pad the damage slightly in all directions to alleviate those effects.

src/scene/workspacescene.cpp

@@ -241,14 +241,29 @@ static void resetRepaintsHelper(Item *item, SceneDelegate *delegate)
     }
 }
 
-static void accumulateRepaints(Item *item, SceneDelegate *delegate, QRegion *repaints)
+static void accumulateRepaints(Item *item, SceneDelegate *delegate, QRegion *repaints, const bool padDamage)
 {
-    *repaints += item->repaints(delegate);
+    if (!padDamage) {
+        *repaints += item->repaints(delegate);
+    } else {
+        const auto padding = 1;
+
+        for (const QRect region : item->repaints(delegate)) {
+            if (region.isEmpty() || region == infiniteRegion()) {
+                *repaints += region;
+                continue;
+            }
+
+            *repaints += QRect{region.x() - padding, region.y() - padding,
+                               region.width() + 2 * padding, region.height() + 2 * padding};
+        }
+    }
+
     item->resetRepaints(delegate);
 
     const auto childItems = item->childItems();
     for (Item *childItem : childItems) {
-        accumulateRepaints(childItem, delegate, repaints);
+        accumulateRepaints(childItem, delegate, repaints, padDamage);
     }
 }
 
@@ -275,11 +290,17 @@ void WorkspaceScene::preparePaintGenericScreen()
 
 void WorkspaceScene::preparePaintSimpleScreen()
 {
+    // if a fractional scale factor is used, pad the damage to avoid visual
+    // glitches due to rounding errors (floating point arithmetic) and/or the
+    // use of GL_LINEAR in the pipeline
+    const auto scale = painted_screen->scale();
+    const bool padDamage = std::trunc(scale) != scale;
+
     for (WindowItem *windowItem : std::as_const(stacking_order)) {
         Window *window = windowItem->window();
         WindowPrePaintData data;
         data.mask = m_paintContext.mask;
-        accumulateRepaints(windowItem, painted_delegate, &data.paint);
+        accumulateRepaints(windowItem, painted_delegate, &data.paint, padDamage);
 
         // Clip out the decoration for opaque windows; the decoration is drawn in the second pass.
         if (window->opacity() == 1.0) {
@@ -314,7 +335,7 @@ void WorkspaceScene::preparePaintSimpleScreen()
     }
 
     if (m_dndIcon) {
-        accumulateRepaints(m_dndIcon.get(), painted_delegate, &m_paintContext.damage);
+        accumulateRepaints(m_dndIcon.get(), painted_delegate, &m_paintContext.damage, padDamage);
     }
 }