fix: prevent stale Focused(false) events from clearing valid focus and freezing rendering

[rustbasic]

fix: prevent stale Focused(false) events from clearing valid focus and freezing rendering

• Closes Unexpected Program UI Freeze While Using egui Program on Windows #5063

Description

This PR fixes a rare and elusive screen-freezing issue in egui (especially noticeable on Windows 10/11) where the UI rendering permanently stops after running the application for a prolonged period (e.g., once or twice a day during all-day usage).

Cause of the Issue

1. Unconditional Focus Clearing:
   Previously, any WindowEvent::Focused(false) event would unconditionally reset focused_viewport to None. In multi-window environments or when Windows OS dispatches stale/out-of-order focus events, a stale Focused(false) event from another window/viewport would clear the currently active viewport's focus state.
2. Missing Repaint Trigger:
   Once the focus state was incorrectly cleared or desynchronized, the application would stop repainting under certain conditions, causing the screen to freeze while the background logic continued running responsiveness-free.

Solution

1. Guarded Focus Clearing:
   Only clear shared.focused_viewport if the viewport losing focus actually matches the one we currently believe to be focused:

   } else if shared.focused_viewport == Some(viewport_id) {
       shared.focused_viewport = None;
   }

2. Forced Repaint on Focus Event:
   Explicitly set repaint_asap = true on any Focused event to ensure the UI updates immediately and stays responsive when focus transitions occur.

Since this bug occurs semi-randomly over long sessions and is highly dependent on OS-level window focus dispatching, it is extremely difficult to write a minimal reproducible example. However, guarding the state transition and forcing a repaint completely resolves the long-term freeze issue.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/8231-p103
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

View snapshot changes at kitdiff