PageControls.syncExcludedViewState() un-hid #modelTypeDropdown whenever
the excluded view was inactive, racing the dropdown's async init. On
pages with a single sub-type (e.g. checkpoints with only "checkpoint"),
init returns early, leaving a bare caret trigger with no label and no
event handlers.
Let ModelTypeDropdown flag data-model-type-available after a successful
init and restore visibility from that flag, so single-type pages stay
hidden while excluded-view transitions still hide and restore an actual
dropdown. Add regression tests for both the single-type and excluded-view
paths.
Will Miao
6d9391672a