gzdoom builder, is there a trick for filling missing visible linedef textures

Welcome to Doomworld.

Two really handy keyboard shortcuts that i use a lot are:

Flood select texture (hold Shift + select) ; which selects all the same textures that are touching.

and 

Flood select texture height (hold Control + Shift + select) ; which selects all the same textures that are touching and the same height. 

I learnt these two shortcuts from watching a Dragonfly mapping stream and then he kindly explained them to me. (So thanks Dragonfly!) and after that i went to the GZDB shortcuts list and made a note of all the new and useful shortcuts available. Getting to know them can really save you a lot of time and headaches. 

Also this is a small time changer : if you want to change a texture, selecting it and going into the menu and then going to "textures" is the slow way. Just put the cursor over the texture/void space and press Control + Edit (right mouse) and you go straight to the "textures" menu. You don't even need to have the texture/void space selected to use this shortcut.

Actually i noticed that i didn't really answer your question very well.....if you want to fill all the missing textures at once you can go to Map Analysis Mode and then Start Analysis. All the areas with missing textures will be listed and you can choose a texture for them or just select the 'default' texture. 

you say you didn't answer it exactly but this shift select actually finds mids and upper and lower that match and allows you to change them all together, it's perfect

also the idea of watching these streams to pick up ideas i never thought of that

i found the stream and twitter, gonna check it out... cheers dude that's really helpful

edit: oh i see you still have the ambiguity of upper, lower, middle, front back etc etc..... HOWEVER, combine the two you mentioned.... shift select, then ctrl-right... that texture replaces all the selected faces without choosing that other bit.... the two shortcuts solve the problem, you're not just skipping to the texture screen, you're choosing a texture contextually for upper lower etc.... nice one