Does not obey QT_SCALE_FACTOR, hence breaks HiDPI

[probonopd]

We are exporting QT_SCALE_FACTOR=2 to switch into HiDPI mode.

However, KWin does not obey this. In fact, looking at the source code, it seems to me it intentionally unsets this variable?

https://github.com/KDE/kwin/blob/9f41691684ffa2f42f09cfb8a24ec23fc146fe27/src/main_x11.cpp#L416

As a result, title bars and other window decorations do not scale; unlike the rest of Qt.

https://phabricator.kde.org/D20234 confirms the suspicion that it intentionally does not follow QT_SCALE_FACTOR since

Qt scaling and kwin on X11 is never going to work so it is disabled.

Why is it working for all of Qt but not for KWin?

[probonopd]

A (very crude and cumbersome) workaround would be to scale all of BreezeEnhanced by the QT_SCALE_FACTOR:

FreeBSD% git diff
diff --git a/breezedecoration.cpp b/breezedecoration.cpp
index 66a09c2..6dbbe0e 100644
--- a/breezedecoration.cpp
+++ b/breezedecoration.cpp
@@ -46,6 +46,9 @@
 #include <QTimer>
 #include <QVariantAnimation>

+#include <QProcessEnvironment>
+#include <QDebug>
+ 
 #if BREEZE_HAVE_X11
 #include <QX11Info>
 #endif
@@ -404,7 +407,17 @@ namespace Breeze
         if( hideTitleBar() ) top = bottom;
         else {

-            top = 22; // probono: Absolute height of the title bar in pixels
+            float scaleFactor;
+            scaleFactor = 1.0;
+            qDebug() << QProcess::systemEnvironment();
+            if(getenv("QT_SCALE_FACTOR")) {
+                qDebug() << "Using QT_SCALE_FACTOR as scaleFactor";
+                QString floatString = qgetenv("QT_SCALE_FACTOR");
+                QTextStream floatTextStream(&floatString);
+                floatTextStream >> scaleFactor;
+            }
+            qDebug() << "Using scaleFactor" << scaleFactor;
+            top = 22 * scaleFactor; // probono: Absolute height of the title bar in pixels

         }

and many other places...

It may even be necessary to binary-patch KWin so that it doesn't unset this variable and BreezeEnhanced gets a chance at reading it.

There must certainly be a better way?

[probonopd]

According to #kwin IRC,

kdeglobals > KScreen > ScaleFactor

for X11, and Wayland will "know automatically". So there is no way to use an environment variable, we must live-patch the kdeglobals file depending on the autodetected resolution? That is very unlike the rest of Qt.

And

[KScreen]
ScaleFactor=2

in ~/.config/kdeglobals does not seem to make any difference for me...

[probonopd]

According to #kwin IRC,

honestly trying to configure kwin without kscreen going is a bad idea user-facing configuration is exposed through the kscreen kcm kcmshell5 kcm_kscreen or systemsettings5 kcm_kscreen

[probonopd]

I was also pointed at

https://github.com/KDE/kscreen/blob/0f5d8dd52b3d02afe7fc57f6be60d69134fe3f42/kcm/kcm.cpp#L362-L385

but I don't know what to make of it.

In helloSystem, we set font sizes to exact point sizes and the only factor that shall influence what gets rendered is QT_SCALE_FACTOR.

[probonopd]

A (very crude and cumbersome) workaround

Probably not as the KWin architects intended, but applying a BREEZE_SCALE_FACTOR that is the same as QT_SCALE_FACTOR in BreezeEnhanced allows us to achieve more or less the desired outcome:

Some fine tuning may definitely be required, and if anyone knows a proper way to get KWin to obey QT_SCALE_FACTOR please let us know.