In https://github.com/atom/atom/pull/15503 we mistakenly assumed
`marker.isValid` accounted only for the validity of the marker. However,
that method returns `false` also for markers that are valid but have
been destroyed. As a result, the editor component was mistakenly not
removing block decorations associated with such markers.
With this commit we will rely on the local `wasValid` variable instead.
If its value is `true`, it means that the block decoration has been
accounted for in the `lineTopIndex` and must, as a result, be cleaned up
in case the marker or the decoration gets destroyed.
This prevents the dummy scrollbars from resetting their position to `0`
when the editor element is moved elsewhere in the DOM (e.g. when
splitting a pane item).
Previously, when trying to use block decorations on non-empty markers,
Atom could sometimes throw an error if such markers ended or started at
a position that was not currently rendered.
In fact, even if we already restricted the decoration query to markers
that intersected the visible row range, markers that were only partially
visible would still be considered for rendering. If, depending on the
`reversed` property, we decided to render the tail or head of the marker
in question and this was outside the viewport, Atom would throw the
aforementioned exception.
This commit addresses the above issue by explicitly ignoring block
decorations that are located on rows that are not yet rendered.
Previously, we were hiding scrollbars when their height/width was 0. On
macOS, however, users can decide to only show scrollbars while
scrolling, which causes Atom to detect scrollbars as being invisible
during measurements. As a result, we were mistakenly setting the
visibility property to `hidden` when this setting was on, thus
preventing users from seeing the scrollbar on scroll.
With this commit we are changing the dummy scrollbar components to only
become invisible when the content is not scrollable rather than when the
scrollbars have zero width or height.
As part of this, we have also renamed the
`is{Horizontal,Vertical}ScrollbarVisible` functions to
`canScroll{Horizontally,Vertically}`, to better express their intent.
This was fixed back in bac9922 and, due to the same reasons, not having
a separate layer causes the "Show scrollbars only when scrolling"
setting on macOS to not work properly.
