[Tryanks]

Summary

Drop gpui-base's window-wide text selection for Markdown; a document owns it instead. The chat transcript is one document whose entries are its messages (at their timeline index). A selectable view outside a transcript, such as a PR comment, is a document of its own.

 press (left)
-  any element in the window → gpui-base clears + begins a gesture (capture)
+  only on the document's area (bubble, hitbox hovered) → begin
 drag / release
-  release ends the gesture only if it bubbles to the root
-    (a drop target or stop_propagation leaves it following the pointer)
+  heard in the capture phase while a gesture is held; nothing can swallow it
 ends of the selection
-  window points; each run highlights the band between them (screen geometry)
+  DocPoint { entry, TextPosition { render path, code line, byte offset } }
+  highlight = run ∩ range; copy = text_between(range)  (same range for both)

crates/ui/src/markdown/
├── selection.rs   # new: SelectionDocument, SelectionArea, hit-testing, gestures, touch
├── nodes.rs       # TextPosition + segment walk; text_between replaces painted-state extraction
├── inline*.rs, math*.rs, render.rs   # each run carries its RunOrigin
└── selection_adapter.rs              # deleted (gpui-base bridge)

What changes for a reader:

• Drag-and-drop no longer leaves a selection. Dragging a thread from the sidebar onto a pane left the gesture active in every chat view (the ui-v2 report). That was the same failure fix(ui): end the text selection before a click stops propagation #595 and the titlebar workaround each patched where they found it.
• Tables read cell by cell along each row. Cells are joined by \t when copied (previously a space), so a pasted table splits into columns in a spreadsheet.
• A drag ending off the text (block padding, the gap under a short line) ends at the nearest line.
• The selection goes when the focus leaves the chat view (the composer is inside it); a press on something outside that takes no focus leaves it, and a press inside the transcript collapses it. A standalone view clears when the focus leaves that view. The chat root now holds a focus handle, so a click on blank chat space takes the focus from the composer, as a click on message text already did. The right-click menu's Copy reads the document.
• Double-click / long press take the word the platform segments, so Chinese splits into words (gpui-base's rule makes every CJK character a word): Core Foundation on Apple platforms, the ICU Windows ships (loaded on first use), Intl.Segmenter in a browser, Android's Java ICU through the activity, and a bundled ICU4X dictionary on Linux only, which has no system segmenter every install carries. Where none answers, gpui-base's rule. Otherwise double and triple click match the window selection: the character under the pointer, the line within a paragraph, no extension by a later drag.
• Each pane selects independently. A PR comment selects within itself and no longer across comments.
• Copy reads messages that were never painted from the timeline, so residency no longer pins selection participants.
• Touch keeps long press → word, handles, an edit menu with Copy / Select All, the menu stepping aside during a pan, and one touch selection at a time.

Removed because only the window selection needed them: stop_click_propagation (now plain stop_propagation), the titlebar's prevent_default/suppress_text_selection (added in 97034b2 for this), the input's TextSelection::clear on long press, WindowTouchSelectionOverlay, the touch-UI registration plumbing, and SelectAllTouched.

Evidence

• Before (main): a press on a drag source, a drag across Hello world, a drop on a target, then the free pointer crossing the text: TextSelection::selected_text = "Hello wor".
  After: a_drag_and_drop_across_the_text_leaves_nothing_selected → "".
• New tests:
  • a_drag_through_a_table_takes_each_cell_whole_before_the_next: a cell wrapped over several rows, dragged to the cell beside its first row → the whole first cell + \t + x.
  • a_drag_ending_off_the_code_lines_ends_at_the_nearest_line.
  • Extraction (nodes.rs): document order, cells, edge-only segments.
  • Word classes for double-click.
• Real app (macOS, a real Claude reply in a throwaway profile):
  • Dragging from mid-line in a code block past the end and into the padding copies from the press onward.
  • A table drag highlights and copies <whole cell>\tOwn.
  • Right-click on the selection → clicking Copy with the mouse copies it, and the highlight stays.
• cargo fmt --check, cargo clippy --workspace --all-targets -D warnings, and cargo nextest run --workspace (1351 passed) on macOS. Not run: Windows by hand, phone/iPad touch on a device, the mobile/Web checks (left to CI).

Tests met in this area:

Test	What happened	Why / where its contract is now
a_scroll_jump_mid_drag_drops_text_left_behind	Driver fixed	It pressed line 40 at y = 862 in an 852 px window, a press no user can make. Now line 38, still in the second root item.
drag_auto_scrolls_at_viewport_edge_not_participant_edge	Ported	Now a_drag_scrolls_at_the_viewport_edge_not_the_message_edge. It caught a real bug in the port: the viewport must be the clip, not the view's bounds. Fails with the old choice.
text_wrapping_after_an_inline_image_paints_on_its_own_rows	Ported	Reads painted runs instead of gpui-base text bounds.
long_press_on_a_message_offers_copy_and_select_all_past_the_viewport, one_touch_selection_at_a_time_and_the_menu_steps_aside_for_a_pan	Ported	Moved to selection.rs. Pan hiding was missing until this port.
long_press_release_keeps_handles_which_drag_the_selection	Rewritten	Drives the document's handles and edit menu. Its final tap moved off the start knob.
drag_copies_exactly_the_highlighted_text, a_drag_ending_off_the_code_lines_copies_what_it_highlights	Deleted	Their oracle was gpui-base's painted projection. Highlight and copy now read one range, so they cannot disagree; literal tests replace them. The midpoint boundary rule has no dedicated test.
extraction_* (nodes.rs)	Replaced	Covered by the text_between tests.
press_on_titlebar_drag_area_does_not_start_selection, click_that_stops_propagation_does_not_leave_selection_following_pointer	Deleted	Same contract as the drag-and-drop test: a press outside the document never selects.
text_bounds_cover_one_rect_per_visual_line	Deleted	Tested a helper that existed only to feed gpui-base.
running_tail_and_selection_pins_are_honored	Renamed	Now the_running_tail_stays_resident; selection pinning is gone.

Added after the first push: the_selection_goes_when_the_focus_leaves_the_text, a_double_click_on_chinese_takes_the_word_the_platform_segments, and in tcode-client the_system_segmenter_finds_chinese_words plus a UTF-16 offset test. The Android segmenter is checked only by CI (no NDK here); its JNI calls were type-checked against jni 0.22.4 on their own.

Merge Danger

Door: two-way

Self-contained in crates/ui. No wire, persistence or protocol change.

Blast Radius: selection

Every Markdown selection path changes (mouse, keyboard Copy/Select All, the context menu, touch), and so does every click handler that used stop_click_propagation. The likeliest regressions are on platforms not exercised here: Windows mouse input and touch on a real device.