Created attachment 186374 [details]
Shows subtitles in bottom subtitle layer aren't selected as expected.

SUMMARY
Selecting all subtitles in different subtitle layers using [Shift] + [Mouse Drag] only selects subtitles in the top layer. Subtitles in the bottom subtitle layer(s) remain unselected. The unselected subtitles in the bottom subtitle layer(s) can be added to the selection via [Shift] + [Mouse Click], but this becomes cumbersome in a real video project with long timelines and multitude of subtitles in more than one subtitle layer. See attached video screen capture. Additionally, selecting everything in the timeline via [Ctrl] + [A] does select all subtitles in all subtitle layers. But, again, this is not useful when you only need to select all subtitles in all subtitle layers.

STEPS TO REPRODUCE
1. Create a new video project and add a short video clip to the timeline.
2. Create subtitles in two different subtitle layers.
3. Select all subtitles in both subtitle layers via [Shift] + [Mouse Drag].
4. Or, select everything in the timeline via [Shift] + [Mouse Drag].

OBSERVED RESULT
Only the subtitles in the top subtitle layer are selected. Subtitles in the bottom subtitle layer(s) remain unselected.

EXPECTED RESULT
It is expected that selecting all subtitles in all subtitle layers via [Shift] + [Mouse Drag] will select all subtitles. This is necessary when moving subtitles to a different point in the timeline.

SOFTWARE/OS VERSIONS
Kdenlive Version: 25.08.2 (AppImage)
Xubuntu (Ubuntu) 22.04.5 LTS

ADDITIONAL INFORMATION
Please find a video screen capture attached to this bug report. It shows the actions described above in easy to follow pace.