Rerender natively rendered text items when DPI scaling changes.

The whole purpose of native rendering is to optimize precisely for a
given pixel grid.

DPI scaling can change when either settings of the current screen are
changed, or when the window is moved to a different screen.

Details:
- add an ItemDevicePixelRatioHasChanged item change event
- detect DPI scaling changes by watching screen (identity) and screen
  config changes
- when DPI scaling changes, recursively send an
  ItemDevicePixelRatioHasChanged signal to all items with content
- when a natively renderet TextItem catches such an event, call
  updateLayout() which automatically picks up the new logical DPI

Task-number: QTBUG-49019
Change-Id: I9f4f8d1a7f2c172ed26c276294ab143161c4a48b
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

1 files changed, 18 insertions, 1 deletions

diff --git a/src/quick/items/qquicktext.cpp b/src/quick/items/qquicktext.cpp
index 3aec464958..aa8f04e55d 100644
--- a/src/quick/items/qquicktext.cpp
+++ b/src/quick/items/qquicktext.cpp
@@ -1455,7 +1455,8 @@ void QQuickText::itemChange(ItemChange change, const ItemChangeData &value)
 {
     Q_D(QQuickText);
     Q_UNUSED(value);
-    if (change == ItemAntialiasingHasChanged) {
+    switch (change) {
+    case ItemAntialiasingHasChanged:
         if (!antialiasing())
             d->font.setStyleStrategy(QFont::NoAntialias);
         else
@@ -1463,6 +1464,22 @@ void QQuickText::itemChange(ItemChange change, const ItemChangeData &value)
         d->implicitWidthValid = false;
         d->implicitHeightValid = false;
         d->updateLayout();
+        break;
+
+    case ItemDevicePixelRatioHasChanged:
+        if (d->renderType == NativeRendering) {
+            // Native rendering optimizes for a given pixel grid, so its results must not be scaled.
+            // Text layout code respects the current device pixel ratio automatically, we only need
+            // to rerun layout after the ratio changed.
+            // Changes of implicit size should be minimal; they are hard to avoid.
+            d->implicitWidthValid = false;
+            d->implicitHeightValid = false;
+            d->updateLayout();
+        }
+        break;
+
+    default:
+        break;
     }
     QQuickItem::itemChange(change, value);
 }