fix(tty): fall back to default dimensions when terminal size is 0x0

[AdamMagued]

Problem

When running Bubble Tea v2 applications on serial consoles, headless setups, or terminals where window dimensions are not yet configured or reported as 0x0 (e.g. stty rows 0 cols 0), term.GetSize returns (0, 0, nil). Bubble Tea sets p.width = 0 and p.height = 0 and passes these dimensions to the renderer and model. Because the viewport height and width are 0, rendered output lines are clipped, resulting in a completely blank screen.

Root Cause

In Program.Run and Program.checkResize:

• term.GetSize(p.ttyOutput.Fd()) can return (0, 0, nil) or non-positive values without returning an error.
• Previously, these dimensions were assigned directly to p.width and p.height without verifying they are positive.
• When s.height is 0 in cursedRenderer, line buffering clips lines, producing no visible content.

Solution

1. Added fallbackDimensions(width, height int) (int, int) helper on *Program with standard terminal defaults (defaultWidth = 80, defaultHeight = 24).
2. If width <= 0, fall back to COLUMNS (checking program environment first, then os.Getenv) if positive, otherwise fall back to 80.
3. If height <= 0, fall back to LINES (checking program environment first, then os.Getenv) if positive, otherwise fall back to 24.
4. Applied fallback in both Program.Run and Program.checkResize.
5. Added unit tests in tty_test.go covering fallback to 80x24, fallback to COLUMNS/LINES, handling invalid or zero env values, and initial window size message dispatch.

Fixes #1828

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.21%. Comparing base (ff51ba4) to head (383d7da).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tty.go	78.26%	3 Missing and 2 partials ⚠️
tea.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1836      +/-   ##
==========================================
+ Coverage   59.91%   60.21%   +0.30%     
==========================================
  Files          25       25              
  Lines        1342     1365      +23     
==========================================
+ Hits          804      822      +18     
- Misses        449      452       +3     
- Partials       89       91       +2     

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[andrinoff]

Please, fix those and it is good to go!

[andrinoff]

This fallback also runs when the output isn't a terminal at all, such as output to a buffer, a pipe, or a Wish/SSH session. Apps that used to receive a 0x0 size there will now get a made-up 80x24. Moving this line inside the if p.ttyOutput != nil block above would limit the fix to the actual bug

[andrinoff]

If the app passed its own environment with WithEnvironment, which is the usual Wish setup, this falls back to the server's own COLUMNS/LINES. Those belong to the wrong machine, so it should only read the program's environment and skip os.Getenv

[andrinoff]

If the app passed its own environment with WithEnvironment, which is the usual Wish setup, this falls back to the server's own COLUMNS/LINES. Those belong to the wrong machine, so it should only read the program's environment and skip os.Getenv

[AdamMagued]

Updated per your review:

1. Restricted fallbackDimensions execution to the if p.ttyOutput != nil block in Run(), preserving 0x0 dimensions for non-terminal outputs (buffers, pipes).
2. Adjusted fallbackDimensions in tty.go so that when p.environ is supplied via WithEnvironment, it only queries the program's environment without falling back to host os.Getenv.
   All unit tests pass.

[andrinoff]

lgtm