tt.nb is what I got from simply starting a new notebook and typing the text is it appears

I tried adding a space in t.nb here, and that does seem to get t.nb acting more like tt.nb, which is what I wanted.

Thanks for letting me know how to work around this undocumented, unpredictable feature of Mathematica.

If you change the cell style from "input" to "code" or convert the cell to InputForm (Shift-Crt-I on Windows) then you can break the likes where you want.

I used "Format->Edit Stylesheet..." to trace down what is the style definition for "Code". This is what I found on Core.nb (using version 10.4.1 at the moment):

Cell[StyleData["Code"],
 PageWidth->Infinity,
 CellMargins->{{66, 10}, {10, 10}},
 Evaluatable->True,
 CellGroupingRules->"InputGrouping",
 TextClipboardType->"Package",
 StripStyleOnPaste->True,
 CellHorizontalScrolling->True,
 PageBreakWithin->False,
 GroupPageBreakWithin->False,
 InitializationCell->True,
 DefaultFormatType->DefaultInputFormatType,
 AutoIndent->False,
 ContextMenu->FEPrivate`FrontEndResource["ContextMenus", "Input"],
 ShowAutoStyles->True,
 ShowPredictiveInterface->False,
 HyphenationOptions->{"HyphenationCharacter"->"\[Continuation]"},
 AutoItalicWords->{},
 AutoQuoteCharacters->{},
 PasteAutoQuoteCharacters->{},
 LanguageCategory->"Input",
 AutoSpacing->False,
 ShowStringCharacters->True,
 NumberMarks->True,
 DigitBlock->Infinity,
 ZeroWidthTimes->True,
 LineBreakWithin->False,
 LineIndent->0,
 CounterIncrements->"Code",
 MenuSortingValue->1450,
 MenuCommandKey->"8",
 FontWeight->"Bold",
 Background->GrayLevel[0.92]]

The front-end is what is forcing the line-breaking, as far as I can tell. This would not surprise me if it were in order to make lines fit within the width of the front-end window; but it is happening even when the line-widths aren't coming close to the window-width, and so that is when it's surprising.

More detail:

1. Everything will be working as I would expect, like every word processor.

2. Then I will be editing a cell, when suddenly the font-end takes one of my semicolon-separated expression-sequences and adds line-breaks after each semicolon.

3. Then I try to delete the line-breaks: I position the text-cursor at the end of the upper line and press the Delete-key, or I position the text-cursor at the beginning of the lower line and press the Backspace-key. This does not succeed in deleting the line-break, though; it rather deletes the first visible character of the lower line or the last visible character of the upper-line.

A still screenshot would be uninformative, as it would just show the expressions formatted in a row as I posted initially, or in a normal-looking column. A screen-video, also showing what keys I'm pressing on the keyboard, would help, but would take 1/2 day for me to produce.

This might even be a feature that Wolfram has gone our of their way to add to Mathematica--it may be helpful when harnessed by the users--, but without my finding its documentation, the "feature" just ends up being erratic behavior from my perspective.

It has happened to be over 5 times, but I don't know how to reproduce it reliably. I do think this "option" is getting saved in the .nb file, though, so we may be able to figure this out if I can catch it in the act again and save the .nb file for analysis here.