helium/linux: middle-click on empty tab strip opens new tab

[saberoueslati]

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Closes imputnet/helium#654.

Manual test below :

Screencast.from.2026-04-26.03-04-37.webm

Modifications made :

Added OnMousePressed and OnMouseReleased to the TabStrip view class and FrameGrabHandle class to handle the middle click event, Added an early return in OnClickedCaption() when the event is a middle-click, which is a Linux-specific blocker that stops the middle click on the empty tab space from reducing the window.

Struggling so far to build Helium on Windows to test the patch there but I'm pretty sure the patch is applicable there without the OnClickedCaption() change

[greptile-apps]

_{Reviews (1): Last reviewed commit: "helium/linux: middle-click on empty tab ..." | Re-trigger Greptile}

[greptile-apps]

Early return bypasses all DE-configured middle-click-caption actions

The early return in OnClickedCaption skips calling GetWindowFrameAction(kMiddleClick, ...) globally, not just for the tab strip. Users who have configured a desktop environment action for middle-clicking the title bar (e.g. lower window, minimize) will silently lose that behaviour even when clicking on window chrome outside the tab strip. The previous code already defaulted to kNone, so most users are unaffected, but this is a broader behavioral change than the PR description implies. It would be worth a comment in the code noting that DE middle-click-caption actions are intentionally suppressed browser-wide.

[saberoueslati]

Yes, they will lose that behavior indeed, and it is intentional, I'll add the comment if the reviewer wants me to