scrollable_mask: Keep rounded scroll-table corners clean at any scroll offset - #3400

…l offset (longbridge#3400)

## Problem

longbridge#3309 rounds the table's first and last rows, which only lines up while
the content sits at its unscrolled position. Once a wide table scrolls
horizontally, gpui's rectangular content mask slices the clipped side
square into the frame's rounded corner notches — the known limit noted
in longbridge#3309.

## Fix

Add `RoundedFrameCover`, a sibling overlay (like `ScrollableMask`)
pinned to the viewport: it fills each corner notch — inside the frame's
rect corner, outside the border arc — with a backdrop color, so the
frame reads as a rounded clip at every scroll offset. The frame's own
border paints after its children, so it stays crisp on top of both the
scrolled content and the covers; nothing is repainted.

`horizontal_scroll_area` installs the cover from the caller style's
corner radii, so markdown tables get it automatically. The element is
exported from gpui-base — `RoundedFrameCover::uniform(radius)`,
`.backdrop(color)`, `.frame_border(width)` — so application frames with
their own rounded scroll viewports can reuse it; `frame_border` grows
the notches out to the border box when the cover is a child of the
bordered element itself.

### Before
<img width="308" height="217" alt="image"
src="https://github.com/user-attachments/assets/780173b3-7225-407b-bd79-6d2ed679a784"
/>

### After
<img width="276" height="273" alt="image"
src="https://github.com/user-attachments/assets/3f75bc6b-2be8-4d60-98b7-36c353b9541d"
/>


## Test

- `corner_notch_paths_stay_inside_their_corner_squares` pins the notch
geometry: each path tessellates to an r-sized square anchored at its
frame corner.
- `cargo nextest run -p gpui-base scrollable_mask` — 17 passed.
- `cargo clippy -p gpui-base --locked -- --deny warnings` / `cargo fmt
-- --check` — clean.
- Verified end-to-end in a consuming application via a local patch:
markdown tables and an application-owned scrollable card keep rounded
corners through horizontal scrolling.

## Description

Prepare v0.7.2 on `stable` from the PRs merged into `main` since v0.7.1,
leaving out every change that breaks existing consumers. `stable`
already carries #3371 and #3373; this adds the remaining compatible PRs
with `git cherry-pick -x` and bumps the workspace to 0.7.2.

### Included

- kit: Add optional gpui-fast backend (#3375)
- tooltip: Make show delay and grace period configurable with
`TooltipDefaults` (#3376)
- docs: Explain recovering focus when the focused element stops
rendering (#3386)
- diff: Add readonly patch viewer (#3378)
- kit: Use one Windows manifest with GPUI Fast (#3380)
- test: Make Progress and TitleBar findable in UI tests (#3383)
- input: Preserve selection, focus ring and context menu actions (#3382)
- diff: Add a Git history example and complete review APIs (#3388)
- text_view: Ignore gestures that stay inside one block plugin (#3390)
- text_view: Round the first and last table rows to the frame radius
(#3309)
- menu: Open a context menu with a long press on touch (#3393)
- text_view: Keep narrow table columns on one line (#3399)
- settings: Keep focus rings inside the list viewport (#3394)
- scrollable_mask: Keep rounded scroll-table corners clean at any scroll
offset (#3400)
- dialog: Add margin_top to AlertDialog (#3404)
- input: Add InputState::add_selection for programmatic multi-cursor
(#3403)
- text_selection: Stop writing selection state on every frame (#3407)
- table: Make the sort icon a labelled button (#3396)
- table: Report visible row ranges of one row or none (#3397)
- dialog: Support configurable entrance animations (#3405)
- docs: Explain the open_dialog builder and footer contracts (#3406)
- dock: Hand the focus to the tab that replaces a closed one (#3409)
- dropdown_button: Add A11y names for the menu trigger (#3410)
- text, resizable: Stop notifying state that did not change (#3418)
- editor: Add on-demand completion commands (#3411)
- table: Clear a right-clicked cell's outline on the next click (#3413)
- tab: Scroll a clicked tab into view with the next one peeking (#3419)
- table: Allow a header row height separate from the body (#3420)
- menu: Focus a context menu before its first frame (#3422)
- text: Preserve logical selection across presentation changes (#3421)
- text: Keep inline HTML mark highlights in Markdown (#3425)
- input: Anchor completion/code-action popups to the current frame's
caret (#3412)
- input: Run custom context menu builders after releasing the input
(#3433)
- input: Keep auto-grow rows fitted when the limits change (#3430)
- text: Paint TextView selection under the glyphs (#3439)

Already on `stable`: #3371, #3373.

### Excluded (Breaking Changes)

- #3432 motion: `Easing::sample` now returns values outside `[0, 1]` for
overshooting curves.
- #3415 tab: new `TabVariant::Folder` breaks exhaustive matches.
- #3402 kit: pointer helpers now keep held modifiers; nested
context-menu behavior changes in tests.
- #3384 test: `TestWindowExt` gains required methods.
- #3302 resizable: `ResizeHandle::on_drag` takes `&T` instead of
`Rc<T>`.
- #3389, #3395, #3398 webview: wrapper API changes, `gpui-wry` →
`gpui-webview` rename, and the GPUI Fast requirement.

#3411, #3405 and #3396 have a Breaking Changes section that reads "None"
and are included. #3376 only deprecates `TooltipRequest::placement`, and
#3397 / #3382 change behavior compatibly, so they are included too.

### Adaptations

- `crates/kit/tests/collections.rs` and `crates/kit/tests/rendering.rs`:
conflict resolutions drop the tests that belong to #3402 and keep each
backported PR's own test.
- `test: Keep backported tests on the v0.7.1 test API`: the menu and
completion tests from #3422 and #3412 used `find_all` and
`pointer_down`/`pointer_up` from #3384/#3402. They now query
`gpui_kit::base::test_support::snapshots` and dispatch mouse events
directly.
- `kit: Prepare v0.7.2`: workspace version bump; `Cargo.lock` only
updates workspace members.

## Public API

No new public API beyond what the included PRs add; each lists its items
in its own `## Public API` section (notably #3375, #3376, #3378, #3388,
#3403, #3404, #3405, #3411, #3420).

## How to Test

- `cargo clippy --workspace --all-targets -- --deny warnings` passes on
Linux.
- `cargo test -p gpui-kit --features "test-support component assets"
--test collections --test menu --test input` passes on Linux.
- The macOS-only `rendering` test (#3439 adds one case, the count is
updated to 11) was not run.

AI-assisted: the backport, conflict resolution and test adaptations were
done with Claude Code.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

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