Three changes:
1. Added box-sizing: border-box to :host of explore-artist-details,
explore-album-details, and artist-details. The light DOM rule
.main-panel > * sets this but shadow DOM may not inherit it.
2. Changed .main-panel to display: flex; flex-direction: column.
In block layout, a height:0 hidden view could affect the
position of the detail view below it. In flex column layout,
the hidden view (flex:0) collapses completely and the active
view (flex:1) fills the remaining space.
3. Changed .main-panel > * from height:100% to flex:1; min-height:0.
In a flex column, flex:1 correctly fills available space while
min-height:0 allows the content to shrink below its intrinsic
size (needed for overflow:hidden to work).