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The Chromium DOM walker clamped frames only to the viewport, so an element scrolled out of an overflow container kept a full frame: describe, await-ui-element and flow assert visible reported it visible, and a flow tap aimed at its centre clicked whatever was painted there instead. Frames are now intersected with the clip of every ancestor on the element's containing-block chain (overflow other than visible, or contain: paint), measured at the padding box less scrollbars. Absolute and fixed boxes escape clippers outside their containing block. A fully clipped element is zero-area, a partially clipped one is framed by its visible part. Scroll containers report that content window as clipFrame, and scroll-to measures a within target against it, so a row cut at a scrollbar is scrolled until it clears the edge.
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The title clearly and concisely summarizes the main change: clipping Chromium frames to their overflow ancestors. |
| Description check | |
The description directly explains the Chromium clipping changes, their effects on visibility, tapping, scrolling, and related verification results. It is clearly related to the changeset. |
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content-visibility: auto|hidden applies paint and layout containment that computed contain does not reflect. Also pin that shadow-root content takes its host's clip.
…es and table rows Transforms and containment do not apply to a non-replaced inline box, nor containment to table rows, so a fixed child there escapes the clip. Also count transform-style: preserve-3d and will-change: translate/rotate/scale/contain.
…nd inline SVG viewports
…g box The picker content escaped every clip, so an option scrolled out of the picker was framed and a tap hit what was painted under it. The picker has no DOM box, so its padding box is placed from its first laid-out child's offsetLeft/Top (which ignore the picker's scroll) and sized from its computed style. Also pin the nested SVG viewport mapping's skew and rotation terms.
…n escapes its select's clips
…ading offsets Reverts the picker padding-box clip: it threw on an SVG picker child and misplaced the picker under CSS zoom and reordered layouts. Pins that only an open select's children escape.
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October 12, 2026 18:29
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Review comments at @packages/tool-server/test/describe-chromium-script.test.ts:
- Around line 977-1384: Add focused clipping regression coverage to the existing
Windows headless-Chrome E2E, rather than creating a separate browser harness. In
the E2E flow that exercises describe and tap behavior, add a clipped fixture and
compare the frame returned by describe with elementFromPoint results in visible
and hidden regions.
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packages/tool-server/src/tools/describe/contract.tspackages/tool-server/src/tools/describe/platforms/chromium.tspackages/tool-server/src/tools/flows/flow-actions.tspackages/tool-server/src/tools/flows/flow-chromium-tree.tspackages/tool-server/src/utils/ui-tree-match.tspackages/tool-server/test/describe-chromium-script.test.tspackages/tool-server/test/flows/flow-scroll.test.ts
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A row scrolled out of an overflow:hidden box must be hidden, and a tap at its painted sibling's described frame must click it. The job now also runs when the Chromium describe walker changes.
… tree Light children of a shadow host took the host's clips, so a menu or dialog slotted into a fixed, absolute or top-layer shadow wrapper read as hidden while Chrome painted it, and rows slotted into a shadow scroller stayed visible past its edge. The shadow tree is now walked first and each slotted node takes the clips at its slot; a slot the walk pruned drops its content.
…ithin is named With frames clipped to their scroller, a row just entering an app-shell list ends flush at the list's bottom, inside the screen, so scroll-to stopped on the first sliver and the next tap failed. Without within, the target is now measured against the smallest other scroller's clip window holding it.
…clip margins - the body clips its own content once the root's overflow is set - a fieldset's rendered legend is not clipped by the fieldset's overflow - an outer <svg> clips at its content box - overflow-clip-margin applies to overflow: clip only when both axes clip - will-change matches the contain keyword, not contain-intrinsic-size Also pins the top and left clip edges, non-uniform clipper scale, table header/footer groups, non-square nested svg viewports and absolute top-layer boxes, which no test held.
A partly clipped Chromium node reports the clip cutting its frame as clipBounds. scroll-to measures the target against that clip and, with no within, anchors the gesture in it, so a sidebar or an unnamed inner list scrolls instead of the page. A whole target flush with its scroller's edge is accepted, and a within wrapper round the real scroller no longer passes on a sliver. Also match Chromium's clipping for will-change tokens, a paint-contained or quirks-mode body, scaled outer svg insets and clip margins, positioned content in a foreignObject, and slots past the depth cap.
scroll-to chased a target that an overflow:hidden card, a clamped section or a fixed bar cuts for good, scrolling the page until the target sat under a header or the loop gave up. clipBounds now comes from the clips a user can scroll, and only for a cut over a pixel: integer client sizes put clip edges a fraction of a pixel inside the true ones, which gave every child flush with a bordered card a clipBounds. Also clip by the body when html or body has any containment, measure a quirks-mode body by its own box (its client size is the viewport's), and frame a display: contents element's text by the flow clip.
With `within` naming an outer pane, scroll-to measured a target against the inner list cutting it but scrolled the pane, carrying the cut along until the pane ran out. A target cut along the scroll axis now anchors the gesture in the cutting window whatever `within` names, and each increment stops where the target's visible start meets the window's: half the window carried a card taller than that past it, and the step passed with the card's top cut. Also match will-change hints in any case and by their -webkit- aliases, frame nothing inside content-visibility: hidden, keep the content of a zero-area box whose overflow Chromium ignores (a scroll-locked body, an inline link), and say in the e2e-add-step remedy that a zero-area frame can be a clip no scroll-to reveals.
This was referenced Oct 25, 2026
… XHTML bodies visible Content slotted into a closed shadow root takes no clip where the page hit-tests it outside its host's clip. A nested <svg>'s viewport follows its own transform. Tag checks ignore XHTML's lowercase tagName, and the walker's Map/Set survive a page's top-level class Map. scroll-to stops once a cut its scroller cannot move stays put. The describe description and the scroll-to docs say how clipping and within behave.
This was referenced Oct 30, 2026
scroll-to anchored a cut target's gesture at the middle of the cutting window, where a nested scroller could take the wheel; it now anchors on the target's visible part. The walker names the axes a scroller cuts (clipAxes), so a scroller spanning the whole screen counts. The closed-shadow escape check allows the same pixel as clipBounds, hit-tests the element's own tree and its descendants, and reads elementsFromPoint through the prototype. A nested <svg> follows translate, rotate and scale too.
… the target The walker now hit-tests, per cut axis, a point whose wheel reaches a scroller that cuts the element (clipAnchors), preferring the point farthest from it, and counts as cutting only scrollers with overflow on that axis. scroll-to wheels there, so an overlay over the target, the target's own scroller, or a scroller nested in the cutter no longer takes the gesture while the step passes. A closed host's child no longer escapes its clips because a fixed or absolute descendant hit-tests outside them.
… target scrolls The walker skips anchors over frames, accepts a scroller that carries the element inside the cutter, tries the innermost cutter first, and falls back to the cut window's centre. A wheel at the anchor that leaves the cut and the tree unchanged (a closed shadow scroller, a wheel listener) no longer ends the scroll: the next increment goes where an uncut target's would.
…tre candidate and retry room
After a wheel scrolls the inner of two cutting lists until it no longer cuts the target, the room against the outer window can equal the old one; that read as a spent anchor and sent the next wheel to the screen centre.
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On Chromium, an element clipped by an
overflowcontainer counted as visible at its full unclipped frame.assert visiblepassed,assert hiddenfailed, andtapreported pass while the click landed on whatever was painted under it. Same fordescribeandawait-ui-element.The DOM walker now clips each frame to its overflow ancestors, the way Chromium paints it. A fully clipped element is zero-area (not visible, nothing to tap); a partly clipped one is framed by its visible part. Where a scroll container cuts it, it also reports that window as
clipBoundsand, per axis, a point where a wheel scrolls it against that scroller asclipAnchors(the window's centre if none is hit-tested).scroll-towheels there, whateverwithinnames, until the scroller no longer cuts it, so a part-shown row of a sidebar or an inner list scrolls that list instead of the page. If the wheel at the anchor leaves the cut in place (a closed shadow root's scroller or a wheel listener took it), the next one goes where an uncut target's would; a cut that survives that too stopsscroll-to. A cut nothing can scroll away (anoverflow:hiddencard, a fixed bar) is measured against the screen, as before, and a target scrolled wholly out of an inner list still needswithinnaming it.overflow:autolist, scrolled below its box, Danger button painted under itassert visible id=hiddenrowtap id=hiddenrowassert hiddenon the rowscroll-to id=hiddenrow within <list>, thentapSide effects:
snapshot cropOnon a partly clipped element crops to its visible part, and "smallest frame wins" compares visible area, as on Android.Docs: the
describedescription says frames are clipped, and thescroll-toreference and e2e skill say a part-shown target scrolls in its own scroller.Clip rules
overflowis notvisible, or under paint containment (contain: paint|strict|content,content-visibility: auto|hidden), at its padding box less scrollbars, scaled under transforms. Paint containment, oroverflow: clipon both axes, extends that edge byoverflow-clip-margin.overflow:hiddencard. A fixed box is clipped with a clipper that contains it (transform,transform-style: preserve-3d, filter,contain,content-visibility, awill-changenaming one of those, a<foreignObject>); as in Chromium, transforms and containment on an inline box, and containment on a table row, do not contain it.content-visibility: hiddenpaints none of its content.showModal()dialog, open popover, fullscreen element) escapes every clip. So does the content of an open customizable select's::picker(select)(all but its leading<button>).<slot>in the shadow tree, so a menu slotted into a fixed or top-layer shadow popup escapes, and rows slotted into a shadow scroller are clipped by it. Content of a slot in a pruned subtree is dropped; a slot past the depth cap leaves its content the host's clips. A custom element's child that no open slot takes (a closed root) gets no clip where Chrome hit-tests it, an in-flow descendant or its shadow tree more than a pixel outside its host's clip.display: an outer<svg>at its content box (scaled under transforms, and the origin of itsoverflow-clip-margin), a<foreignObject>at its rect, a nested<svg>at its viewport mapped through the parent's screen CTM and its own transform.display: contentsand table rows, row groups and columns. A fieldset's rendered<legend>is not clipped by the fieldset.Verification
main's. Everydescribeframe is compared withelementFromPointsampling.mainframes it and the click hits Danger; this branch reports no visible frame.<dialog>in anoverflow:hidden; transformcard, popover in awill-change: transformclipper: visible and clicked, as onmain. A plain fixed button in the same card stays hidden, matching Chrome.showModal()dialog, a fixed popup and an absolute popup, each in anoverflow:hiddencard, are visible as onmain; rows slotted into a shadowoverflow:autoscroller (theion-contentshape) are clipped wheremainframed them past its edge. 0 mismatches on 8 shadow pages.overflow: clipmargins,contain: paint,content-visibility, a padded outer<svg>at 2x and underoverflow: clip, nested<svg>viewports, fixed and absolute buttons in a<foreignObject>, a thick-bordered fieldset legend, a contained body or html, a quirks-mode body,display: contentswithposition, a slot past the depth cap,will-change: transform-origin: each frame ends where Chrome stops painting.scroll-to, thentap, on an app shell (header,overflow:autolist, footer) passes 23/23 rows with and withoutwithin, and taps BUY-7 (maintaps the footer) with the list unnamed. It also passes on an off-centre side list (the list scrolls, the page stays), withwithinnaming the page pane round a part-shown widget row (mainscrolls nothing and passes; before this fix the pane ran to its end), on 170px cards in a 200px list (the card lands whole and its Edit button is clicked), and withwithinnaming a padded wrapper (29/29 rows). An already visible first or last sidebar link, a Buy button flush with a bordered card, a row cut by a clamped section and a button cut by a fixedoverflow:hiddenbar pass in about 270 ms with nothing scrolled and the click on the target.mainpasses the same flows too, except on the unnamed list, where its tap hits the footer.windows-chromium-e2e.mjsnow checks a clipped row in CI: passes here, fails onmain.overflow:hiddencard is visible and tapped, as onmain; rows slotted into a closed plain wrapper, or children of a light-DOM custom element, in a scroller stay clipped. Transformed nested<svg>s, an XHTML body and a page with a top-levelclass Mapmatch Chrome too.mainscrolls the inner one and passes with the target still cut). A target cut by a scroller spanning the whole viewport height scrolls into view with and withoutwithin. Rows of a light-DOM custom element in a 400.2px-wide clipper stay clipped (mainframes row 1 under the header, so its tap clicks Logout).preventDefaultwheel listener lands whole, as onmain. A row cut by a list inside a scrolling article lands whole (mainscrolls nothing); a 160px target cut by two nested lists ends with 141 rows painted (main63).scroll-toon a row whose visible part sits under a fixed tab bar, on a target that scrolls itself (a list, a<textarea>), on a card in anoverflow-x:autocarousel under a scrolling pane, and on a scrollable panel inside a list: the target lands whole and a followingtaphits it.describe-chromium-script.test.tsandflow-scroll.test.tsfail onmain's code. The rest pin that rows, body,transform: scaleand top-layer boxes are not over-clipped.