Selecting a tuplet crossing a page creates a weird UI issue.
Reported version
3.4
Type
Graphical (UI)
Frequency
Once
Severity
S4 - Minor
Reproducibility
Always
Status
needs info
Regression
No
Workaround
No
Project
0) Open a score in MuseScore.
1) Create a tuplet at the end of a page/page break so that the end of the tuplet is the end of the measure.
2) Select the last beat of the tuplet and any preceding beat.
Expected: Blue selection box highlights and selects only the selected notes in the tuplet.
Actual: Blue selection box extends to the next page, sometimes overflowing to the entire measure, if the next measure consists of only a measure rest. 
Comments
Sample score needeed. Tuplets generally can't cross measure boundaries (in MuseScore), if they do this would point at a corrupted score
In reply to score needeed. Tuplets… by Jojo-Schmitz
The tuplets aren't crossing the measure, only the selection box that shows what is selected overflows; not in playback or editing do the tuplets act like they cross the barline.
In reply to The tuplets aren't crossing… by [DELETED] 31341777
In fact it doesn't have to be a tuplet and it doesn't need to involve a page break, just a system break.
Fill the last bar of a system with quavers, highlight the last quaver, press Shift+left arrow. The selection box extends from the penultimate quaver into the next bar (on the next system)
However, if you start by clicking on the penultimate quaver and extend the selection to the right with Shift+right arrow only the last two quavers are selected (as expected).