M3: Add middle-click autoscroll on Linux (#111)

* M3: add middle-click autoscroll

Co-Authored-By: Codex <noreply@openai.com>

* M3: keep middle-click autoscroll toggled after release

Keep scrolling enabled after releasing the wheel, including when the
pointer moves before release. Anchor speed to the initial press event,
so movement later in the same frame cannot shift the starting point.

Cover toggle behavior and increasing speed with distance in both views.

Co-Authored-By: Codex <noreply@openai.com>

* M3: increase middle-click autoscroll speed

Double scrolling speed at the same pointer displacement and raise the
speed cap proportionally, preserving the dead zone and click toggle.
Keep the sustained-scroll fixture visible at the faster response.

Co-Authored-By: Codex <noreply@openai.com>

* M3: match Chromium middle-click scrolling

Use a 15-point dead zone and Chromium's nonlinear distance-to-speed curve
in the document viewer and Organize Pages. Request continuous redraws so
scrolling follows elapsed frame time and preserves fractional movement.
Keep click toggling, cancellation, and the existing marker unchanged.

Cover reference speeds, pathological inputs, pause/resume, and consistent
travel at 30/60/120/144 fps with different display scales and document zoom.

Co-Authored-By: Codex <noreply@openai.com>

* M3: leave README unchanged in autoscroll PR

Remove the feature bullet so README matches upstream main. Keep the
feature explanation in the pull request description.

Co-Authored-By: Codex <noreply@openai.com>

* M3: limit custom middle-click scrolling to Linux

Return before handling input on other targets so their existing
middle-button behavior, widgets, cursor, and Escape shortcuts stay intact.
Restrict the autoscroll UI regressions to Linux and add unsupported-platform
input coverage plus a non-Linux UI regression for the native CI matrix.

Co-Authored-By: Codex <noreply@openai.com>

* ROADMAP: drop the log line about merging the rename

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Codex <noreply@openai.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

parity/acrobat-features.toml

@@ -887,11 +887,11 @@ title = "Automatic scrolling"
 area = "B"
 tier = "P2"
 milestone = "M3"
-status = "planned"
+status = "partial"
 commands = []
 tools = []
-evidence = []
-notes = ""
+evidence = ["crates/ui-egui/tests/control.rs::middle_click_autoscroll_latches_has_a_dead_zone_and_scrolls_both_directions", "crates/ui-egui/tests/control.rs::organize_pages_supports_autoscroll_without_selecting_or_reordering_pages", "crates/ui-egui/tests/control.rs::moving_before_middle_button_release_keeps_scrolling_without_starting_page_tools", "crates/ui-egui/tests/control.rs::farther_from_the_click_scrolls_faster_in_the_viewer_and_page_grid", "crates/ui-egui/tests/control.rs::autoscroll_uses_the_initial_click_position_when_input_arrives_in_one_frame", "crates/ui-egui/tests/control.rs::autoscroll_uses_elapsed_frame_time_and_preserves_fractional_motion_in_both_views"]
+notes = "Linux-only middle-button vertical autoscroll in the document and Organize Pages; other targets retain their existing middle-button input behavior. Click to toggle, release keeps scrolling on, and a Chromium-style nonlinear curve increases speed with vertical screen distance from the initial click. A 15-point dead zone pauses immediately. Continuous redraws use elapsed frame time and preserve fractional movement, tested at 30/60/120/144 fps. Cancels on another middle click, Escape, other pointer/wheel input, focus loss, tab changes and dialogs. Driven headlessly through the opt-in UI control channel (ui.click/ui.move/ui.drag). Keyboard-started timed scrolling is not implemented."
 
 [[feature]]
 id = "view.zoom-in-out"