Only editors inside #root-view #panes are tracked as the most recent active editor. Otherwise the file finder tries to open the selected file in its own mini editor.
Using percentage dimensions to absolutely position the panes on screen where they need to be located. Flexbox would have been nice, but unfortunately I could not work around what seem to be bugs (or at least major inconveniences) in its current implementation.
Switching back to the new flexbox and re-introducing a scrollable-content node that the flexbox is applied to. This has no height settings so it grows to contains the gutter and lines. The editor outside of it has a width / height of 100% of its container plus overflow-y of scroll. Trying to apply flexbox AND a 100% height + scroll all to the editor causes display artifacts.
For now, I'm removing the flexbox treatment of #app-vertical because the file finder just needs to overlay the bottom and absolute positioning is simpler. I'm also using requireStylesheet to load the file finders CSS instead of a link tag.
