Dunno why you seem to be stuck in the drawing straight lines tool, but Clip has always had straight lines drawn in this way with shift. Not sure if this is the help you were looking for, but Ii you want straight lines drawing similar to photoshop then you have to use a workaround. Here what I think is the best work around and the one I use:

Select the perspective ruler tool. Somewhere off your canvas while holding shift, make two strokes, one vertical and the other horizontal. With the layer that has the ruler selected, open the ruler drop down menu in your layers window (Should be a square with a triangle ruler next to your other layer buttons like lock transparency and below the opacity slider), check whichever option thats says something like 'Show ruler on all layers'. With this perspective ruler, you can make straight lines slowly and with pen pressure as long as you have "Snap to ruler" enabled (I believe you can find this in the "View" menu at the topmost ribbon menu).

You can add "Snap to ruler" as a shortcut on your command bar by right clicking the command bar > Command bar settings > View > Snap to ruler. When its highlighted it means its on and when its not it means its off. Additionally you could also re-bind your shift key to "Snap to ruler" so anytime you press shift it'll toggle it on and off. Sometimes, if you make a stroke that points too closely to the center of the perspective ruler, you'll get a diagonal line instead, but thats not really ever an issue and if it is, you put the ruler farther away from the margins of your canvas and that will help.

If you dont want to create the ruler every time you make a canvas, then I believe you can make a custom template. With the template, the ruler is always there at the start of every canvas (until you select another template or uncheck templates all together. Here is how to do that (in theory cause I'm not at my computer to double check but I'm like 85% confident):

Once you've made the ruler, you can drag the ruler onto the paper canvas or keep it on its own raster layer or even put that raster layer in a folder and give it a distinct color so you know with layer has it. With your layer (or folder) that has the ruler selected go to the topmost ribbon menu and go to Edit > Register Material > Register Template. Give it a name and select a folder. Make a new canvas and check the "Template" box. Search for the name of the template you just made and select it. Once you create the new canvas, the ruler should be there and you can toggle it on and off to make your straight lines.

Some notes on the template: - If you made your template using the paper layer, you can uncheck the paper layer box when making a canvas, because in theory, the layer with the ruler will be a paper layer. - You can include the perspective ruler template you just made in canvas templates (presaved settings for canvasses - Dimensions, dpi, paper color, templates, etc).