Allow TitleBar to not respond to onDoubleTap to fix visible delay on clicks

[artdedeco]

The new TitleBar control, which replaces the old NavigationAppBar, currently uses a GestureDetector to respond to pan (drag) and double tap/click events. However, GestureDetector has a known, currently unresolved, issue where it adds a 300ms delay to single tap events when you listen to onDoubleTap, thus rendering TitleBar unusable in any desktop, mouse & keyboard environment.

There are a few different avenues for fixing this on the library side, notably: 1) depend on a lower level gesture detection abstraction and choose our own algo for differentiating between single and double clicks - perhaps one better suited for mouse, given the target audience of the library; or 2) redesign the control so that gesture detection does not apply to the whole thing (which would have the added benefit of better aligning to Windows UX standards -- drag should not work when clicking on app icon or caption buttons, for example).

Given that both of those options would not only require non-trivial amount of work, but also a discussion phase, I'm proposing this change as a quick mitigation, so that at the very least it unblocks teams currently blocked by this, such as mine. The change is a one liner: from onDoubleTap: () => onDoubleTap?.call() to onDoubleTap: onDoubleTap; the idea being that if the caller provides a null func to onDoubleTap, the onDoubleTap callback from GestureDetector is never registered.

I categorized the commit as refactor(perf) following precedence on the changelog file, and given that the change does not alter the contract nor does it cause any externally visible change in behavior for the caller (beside the improved perf).

Pre-launch Checklist

• I have updated CHANGELOG.md with my changes
• I have run "dart format ." on the project
• I have added/updated relevant documentation

[gemini-code-assist]

Code Review

This pull request refactors the TitleBar component to pass the onDoubleTap callback directly instead of wrapping it in an anonymous function. This prevents the gesture detector from listening to double taps when no callback is provided, avoiding a 300ms delay on single taps. There are no review comments, and I have no feedback to provide.

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[artdedeco]

I just noticed #1298 was responsible for adding this (support for double click callback) - I had thought it was a more fundamental design decision to support it from the start; Instead, it seems more like it was just an oversight in that implementation. I guess that makes this fix I'm proposing acceptable even long-term.