This can be seen in all the examples and autotests which use the shared LauncherList.qml . It's due to an early return in QQuickWindowPrivate::deliverMouseEvent. The fix is https://codereview.qt-project.org/#/c/211616/
The reason this got into the RC is that somebody decided to start testing boot2qt in CI, some time last week (terrible timing: it should have been started right after 5.10 branching, not so late). The result was an error in qtbase:
which blocked the 5.10 qtdeclarative branch in CI for several days (at least from Nov. 17 - 21, perhaps longer). Nobody seemed to be on top of that, so I fixed it: https://codereview.qt-project.org/#/c/211734/ Then I kept asking for qt5.git to be updated to include the fix, but that didn't get done, either.
That prevented the fix for this bug from getting integrated before the 5.10.0 branching was done. And due to the tyranny of the release schedule, it's now quite likely that the bug will ship, too.