Summary

• On Linux, middle-clicking a tab to close it also triggered a paste of the primary selection into the editor behind it, because the event bubbled to the editor's mouse_up() handler
• Added cx.stop_propagation() in the on_aux_click handler in pane.rs after closing the tab, matching the pattern already used by the editor's own middle-click handler in element.rs:1037

Closes #47381

Test plan

• cargo test -p workspace passes (124 tests, 0 failures)
• Select text in editor (filling primary selection), middle-click a tab to close it → tab closes, no paste in the editor behind
• Middle-click inside the editor area → still pastes from primary selection as expected

Release Notes:

• Fixed middle-click on a tab to close it also pasting into the editor on Linux.