[Webkit-unassigned] [Bug 213851] New: Container in RTL mode shows undesired space on right if `scrollLeft` is accessed

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=213851

            Bug ID: 213851
           Summary: Container in RTL mode shows undesired space on right
                    if `scrollLeft` is accessed
           Product: WebKit
           Version: Safari 12
          Hardware: Unspecified
                OS: macOS 10.14
            Status: NEW
          Severity: Minor
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: paulgschwendtner at gmail.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Assuming a user has scrollbars set to `Always Show up` (as per macOS System preferences).

Consider a `div` set to `dir=rtl` with content that expands the containers width and a vertical scroll bar shows
purely because of the displayed horizontal scroll bar.

If the div/scrollable container is inserted dynamically through `document.appendChild` after `container.scrollLeft` has
been accessed in any way, undesired spacing is displayed on the right. The space seems to be of 15px and represents
the usual LTR vertical scrollbar.

Note for clarity: This only seems to surface when the container is inserted dynamically, and if `scrollLeft` is accessed. An
requirement for reproduction also seems that the vertical scroll bar is caused by the horizontal scroll bar showing up.

A small reproduction can be found here: https://codepen.io/DevVersion/pen/Rwrjqor:

 1. Open the CodePen in Safari macOS
 2. Make sure that `Scroll bars` are always enabled as per MacOS General preferences
 3. Observe the space on the right side (while the scroll bar is on the left)

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