Hi here is a way to create breaklines in screenshots using stickers and a combination of other tools in the app.
One example would be a long list and use cut tool to remove desired area, add position break line, the ctrl-c ctrl-v and in newly created tab use crop tool to eliminate the break line extensions and save. After a couple it gets easy.
I chose to use 14" and 24" (I know I use inches not px but Inkscape is just a button press) which seem to cover a full 1080x1920 full screen shot thus any as you crop the excess. Had issue with different lengths when stretching they distorted and this gives consistent look across all shots. In image I have ten stickers which covers both horizontal and vertical break lines. Just put .svg file on Desktop, go to settings and add, navigating to Desktop. This stores in in /home/user_name/.local/share/ksnip/ksnip and file on Desktop is no longer needed. (save if you like in your favorite location)
If interested you can download the double lines from this link below by right clicking on each one and save image as. For the dashed lines you can create in Inkscape. Did these while was member on Github but left. Did not upload the dashed line versions as easy enough to create in Inkscape.
Image below shows lines and cropping of vertical but same with horizontal. Maybe this information might help if you want to create a custom sticker, logo for your screenshots. Explore you will find ctrl-c ctrl-v and shift-ctrl-v can be used to create some nice screenshots.
Edit: Located dashed line dimensions for link instructions
All using 6px dash wit 6, 12, 24, 48 px white area.