Cursor misposition and text overlap in QGIS expression editor on Ultramarine Linux KDE Plasma (text after = overlays previous chars)

[Drew1618-cell]

What is the bug or the crash?

When typing expressions in QGIS expression editor/field calculator (e.g., "function"12345"), characters like numbers/symbols after quotes/names appear overlaid at the position of the previous character. Backspacing moves the cursor abnormally left behind existing text, making editing impossible.

• Steps tried: Several different combinations of the following environment values QT_QPA_PLATFORM=xcb;QT_STYLE=fusion;QT_AUTO_SCREEN_SCALE_FACTOR=0;QT_SCALE_FACTOR=1;QT_FONT_DPI=96 qgis (issue persists)
• Installation: Both Flatpak and DNF repo versions affected

Current behavior
Text overlays previous chars (e.g., ="12345" stacks on "function"); cursor mispositions on backspace.

Expected behavior
Normal sequential text insertion and cursor movement without overlap.

Steps to reproduce the issue

1. Open QGIS on Ultramarine Linux KDE Plasma.
2. Go to any expression editor (Layer Properties > Fields > Expression..., Field Calculator, etc.).
3. Type something like `"function"="12345".
4. Observe text after first few chars after the = sign (e.g., ="12345") overlapping previous position; backspace jumps cursor left.

Versions

QGIS version: 3.44.7-Solothurn
QGIS code branch: Release 3.44 (https://github.com/qgis/QGIS/tree/release-3_44)

Libraries:

• Qt version: 5.15.18
• Python version: 3.14.2
• GDAL version: 3.11.5 (Eganville)
• PROJ version: 9.6.2
• EPSG Registry database version: v12.013 (2025-05-26)
• GEOS version: 3.14.1-CAPI-1.20.5
• SQLite version: 3.50.2
• PDAL version: 2.9.3
• PostgreSQL client version: 18.0
• SpatiaLite version: 5.1.0
• QWT version: 6.2.0
• QScintilla2 version: 2.14.1
• OS version: Ultramarine Linux 43 (KDE Plasma Edition)
• KDE Plasma version: 6.5.5
• Kernel: 6.18.9-200.fc43.x86_64
• GPU/Driver: AMD/ATI Lucienne (rev c1)

Active Python plugins:

• MetaSearch: 0.3.6
• db_manager: 0.1.20
• grassprovider: 2.12.99
• processing: 2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

• Issue consistent across expression builders in QGIS.
• No problems in other Qt apps or KDE text editors.
• Both Flatpak and native DNF installs broken—likely KDE/Qt-Plasma font kerning/cursor sync issue.
• QGIS LTR.

[ViperMiniQ]

I also use KDE Plasma and I don't have that problem running QGIS on a Wayland session (although, I compile my own version, so the issue could be the packager).

Try a Qt6 version.

[Drew1618-cell]

@ViperMiniQ Thanks. I tried using Qt6 but no success. Tried compiling 3.44/3.99 with all Qt6 devels installed.

Issues:

• libspatialindex 2.1.0 (F43 pkg) incompatible (ABI break, CMake blocks >=2.1)
• Multiple Qt6 modules (SerialPort, Positioning, Core5Compat, SqlPrivate) missing devel configs in Fedora 43 (the base for Linux Ultramarine) repos
• 3.44/master still detects system spatialindex and fails; internal fallback not triggered easily

Ended up with roughly 80% configure success but generate fails on NOTFOUND vars.
Any Fedora-specific tips or should I wait until there is a packaged Qt6 LTR available?

[ViperMiniQ]

I don't have such issues on Fedora.

For spatial index, use -DUSE_INTERNAL_SPATIALINDEX=ON -DWITH_INTERNAL_SPATIALINDEX=ON
As for the rest, follow the instructions and install the packages mentioned in https://github.com/qgis/QGIS/blob/master/INSTALL.md#310-on-fedora-linux

If there is something specific Ultramarine OS lacks, I am afraid I cannot help with that.

[Drew1618-cell]

That worked to successfully built QGIS 3.99.0-master with Qt6 on Ultramarine Linux 43. DWITH_INTERNAL_SPATIALINDEX=ON and the instructions in the link solved the compiling issue. However, the original issue of cursor misposition and text overlap still persist in this version too.​

How I Solved Build Issues

• Qt6SerialPort etc.: sudo dnf install qt6-qtserialport-devel
• spatialindex 2.1.0 block: -DWITH_INTERNAL_SPATIALINDEX=ON bundles it (system 2.1.0 ABI-broken for QGIS).
• Full cmake (in build-qt6/):
  cmake .. -DWITH_INTERNAL_SPATIALINDEX=ON -DBUILD_WITH_QT6=ON -DWITH_SERVER=OFF
  -DWITH_QTWEBENGINE=ON -DWITH_BINDINGS=ON -DWITH_PYTHON=ON -DWITH_DESKTOP=ON
  -DWITH_3D=ON -DWITH_ANALYSIS=ON -DWITH_PDAL=ON -DENABLE_MODEL=NO
  -DCMAKE_BUILD_TYPE=RelWithDebInfo -G Ninja
• ninja -j$(nproc)
• ./output/bin/qgis # Runs!
• Added @development-tools ninja-build; -DWITH_SERVER=OFF skipped missing fcgi-devel.​

Bug Update
Cursor/text overlap still reproduces in fresh master Qt6 build (e.g., "function"="12345" overlays after "function"; backspace jumps left).

Tested:

• Start with default Wayland Plasma 6.5.5
• Start with X11 instead via QT_QPA_PLATFORM=xcb ./output/bin/qgis
• Start with KWIN_FORCE_SW_CURSOR=1 ./output/bin/qgis instead

Hypothesis:
I suspected that the issue lies in QScintilla2/Qt6-Plasma text rendering.