base: Stop auto scroll once the text selection gesture has ended

[madcodelife]

Summary

• Fix markdown (and any other TextView) scrolling on its own after a single click: click on text, then move the pointer toward the top or bottom edge of the viewport, and the view starts auto scrolling and keeps going.
• Root cause: WindowSelectionState::end() keeps the anchor after mouse up so a shift-click can extend it, and only clears is_selecting. base: Move TextView to gpui-base #2881 moved the auto scroll call out of update_impl (which has the is_selecting guard) into update_in_window, so update_auto_scroll ran on every pointer move with a live anchor. Once the pointer was in the edge zone it started the anchor auto scroll, and the synthesized ScrollWheelEvent re-entered the same path and kept it alive.
• Guard update_auto_scroll on is_selecting, restoring the pre-base: Move TextView to gpui-base #2881 behavior: only a live drag may scroll.
• Add the regression test pointer_moves_after_a_click_do_not_auto_scroll: begin a gesture on text, end it, then drive update_in_window past the viewport edge and assert auto scroll stays idle.

The fix and the test were written with Claude Code.

Test plan

• pointer_moves_after_a_click_do_not_auto_scroll fails before the fix and passes after.
• cargo test -p gpui-base --lib text_selection — all 49 tests pass.
• cargo test -p gpui-component --lib text::window_selection — all 51 tests pass, including drag auto scroll in both directions.
• cargo clippy -p gpui-base --all-targets -- --deny warnings and cargo fmt --check are clean.

🤖 Generated with Claude Code