Fix Qt6 menus randomly not displaying

[Cordtus]

Fixes #624

Problem

When building LazPaint with Qt6 widgetset (--ws=qt6), menus would randomly fail to appear in the main menu bar. The "Image" menu was particularly affected, but other menus could also be missing on any given launch (roughly 30-70% failure rate across multiple launches).

This issue was tested and reproduced on Arch Linux with qt6pas 6.2.9, FPC 3.2.2, and Lazarus 3.8.

Root Causes and Fixes

1. TActionList.ActionByName race condition with Qt6

During early initialization, TActionList.ActionByName appears to have timing issues with Qt6 that cause it to fail to find actions that are actually present. Debug logging confirmed all 177 actions were loaded, but ActionByName would sometimes return nil.

Fix: Added FindActionByNameDirect helper function that iterates directly over ActionList.Actions[] instead of using the hash-based lookup.

2. Menu visibility not explicitly set

Qt6 sometimes fails to display TMenuItems even when they're correctly populated with all their child items.

Fix: Added explicit Visible := true for all used menus in the Apply procedure.

3. Main menu bar widget needs refresh

Even with correct menu data, Qt6's main menu bar widget wouldn't always render all items.

Fix: Added menu detach/reattach sequence with ProcessMessages calls after menu population to force Qt6 to rebuild the menu bar widget:

Self.Menu := nil;
Application.ProcessMessages;
Self.Menu := MainMenu1;
Application.ProcessMessages;

Testing

Before this fix: 2/6 launches had the Image menu (~33% success rate)
After this fix: 6/6 launches had the Image menu (100% success rate)

The fix has been tested repeatedly with consistent results.

Changes

• lazpaint/umenu.pas: Added FindActionByNameDirect helper, replaced ActionByName calls, added explicit visibility setting
• lazpaint/lazpaintmainform.pas: Added menu bar refresh after m.Apply

[Cordtus]

Upon further testing there are some issues this has created.

• I feel like the performance is quite degraded, but this could just be the fact that my pc is quite old, with a 4th gen i5.
• I noticed just now that the entire selection of menus intermittently does not display at all, but I recall this happening quite a lot for other applications too so it may be completely unrelated.