Originally the `PDFSidebarResizer` class was slightly larger, since the code used to contain e.g. feature testing for older (and no longer supported) browsers.
Given that there's some amount of overlap, when it comes to what DOM-elements and state that these classes need, it now seems reasonable to simply move the sidebar-resizing into the `PDFSidebar` class.
For the MOZCENTRAL build-target this patch reduces the size of the *built* `web/viewer.js` file by just over `1.1` kilobytes.
Given that the GV-viewer isn't using most of the UI-related components of the default-viewer, we can avoid including them in the *built* viewer to save space.[1]
The least "invasive" way of implementing this, at least that I could come up with, is to leverage import maps with suitable stubs for the GV-viewer.
The one slightly annoying thing is that we now have larger import maps across multiple html-files, and you'll need to remember to update all of them when making future changes.
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[1] With this patch, the built `viewer.js` size is 391 kB and `viewer-geckoview.js` is 285 kB.
