Fix Qt6 rendering: exclude from DIRECTUPDATE/QUICKUPDATE optimizations

[Cordtus]

Problem

When using LazPaint with the Qt6 widgetset, users may experience sluggish rendering and UI glitches during editing or zooming operations.

Root Cause

The uimageview.pas file defines two rendering optimizations:

{$IF defined(LINUX) and not defined(LCLqt5)}{$DEFINE IMAGEVIEW_DIRECTUPDATE}{$ENDIF}
{$IF not defined(DARWIN) and not defined(LCLqt5)}{$DEFINE IMAGEVIEW_QUICKUPDATE}{$ENDIF}

These optimizations:

• IMAGEVIEW_DIRECTUPDATE: Bypasses Qt's paint queue and renders directly
• IMAGEVIEW_QUICKUPDATE: Forces immediate Update() after InvalidateRect()

The original code (from 2020, commit b80b767) correctly excluded Qt5 from these optimizations, but Qt6 was not yet supported by Lazarus at that time and was not excluded.

Why Qt5/Qt6 Need Different Handling

Qt's paint system expects all painting to go through paintEvent() handlers via the event loop. From Qt documentation:

update() does not cause an immediate repaint; instead it schedules a paint event for processing when Qt returns to the main event loop. This permits Qt to optimize for more speed and less flicker. Calling update() several times normally results in just one paintEvent() call.

Qt6 maintains the same event-driven paint model as Qt5. In fact, Qt6 went further by deprecating repaint() entirely, as synchronous painting "does not work well on modern systems."

The direct update patterns that work on GTK conflict with Qt's event coalescing and can cause:

• Race conditions between paint events
• Double-painting
• Visual tearing and artifacts

Solution

Add and not defined(LCLqt6) to both conditional defines, matching the existing Qt5 exclusion:

{$IF defined(LINUX) and not defined(LCLqt5) and not defined(LCLqt6)}{$DEFINE IMAGEVIEW_DIRECTUPDATE}{$ENDIF}
{$IF not defined(DARWIN) and not defined(LCLqt5) and not defined(LCLqt6)}{$DEFINE IMAGEVIEW_QUICKUPDATE}{$ENDIF}

References

• QWidget Class | Qt 6.10.1 - Documents update() vs repaint() behavior
• QPaintEvent Class | Qt 6.10.1 - Paint event handling
• Qt Contributor Summit 2019 - Refurbishing Qt Widgets - Discusses repaint() deprecation
• Original Qt5 exclusion: commit b80b767 ("optimize for qt5", 2020)

Testing

Built and verified application launches correctly on Arch Linux with Qt6Pas 6.2.9, FPC 3.2.2, Lazarus 3.8.