[vim] Add support for xoffset and yoffset options for popup

Close https://github.com/junegunn/fzf.vim/issues/942

README-VIM.md

@@ -198,9 +198,11 @@ When `window` entry is a dictionary, fzf will start in a popup window. The
 following options are allowed:
 
 - Required:
-    - `width` [float]
-    - `height` [float]
+    - `width` [float range [0 ~ 1]]
+    - `height` [float range [0 ~ 1]]
 - Optional:
+    - `yoffset` [float default 0.5 range [0 ~ 1]]
+    - `xoffset` [float default 0.5 range [0 ~ 1]]
     - `highlight` [string default `'Comment'`]: Highlight group for border
     - `rounded` [boolean default `v:true`]: Use rounded border
 
@@ -291,10 +293,12 @@ The latest versions of Vim and Neovim include builtin terminal emulator
 
 ```vim
 " Required:
-" - width [float]
-" - height [float]
+" - width [float range [0 ~ 1]]
+" - height [float range [0 ~ 1]]
 "
 " Optional:
+" - xoffset [float default 0.0 range [0 ~ 1]]
+" - yoffset [float default 0.0 range [0 ~ 1]]
 " - highlight [string default 'Comment']: Highlight group for border
 " - rounded [boolean default v:true]: Use rounded border
 let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }

doc/fzf.txt

@@ -1,4 +1,4 @@
-fzf.txt	fzf	Last change: February 3 2020
+fzf.txt	fzf	Last change: February 6 2020
 FZF - TABLE OF CONTENTS                                            *fzf* *fzf-toc*
 ==============================================================================
 
@@ -217,9 +217,11 @@ When `window` entry is a dictionary, fzf will start in a popup window. The
 following options are allowed:
 
  - Required:
-   - `width` [float]
-   - `height` [float]
+   - `width` [float range [0 ~ 1]]
+   - `height` [float range [0 ~ 1]]
  - Optional:
+   - `xoffset` [float default 0.0 range [0 ~ 1]]
+   - `yoffset` [float default 0.0 range [0 ~ 1]]
    - `highlight` [string default `'Comment'`]: Highlight group for border
    - `rounded` [boolean default `v:true`]: Use rounded border
 
@@ -306,10 +308,12 @@ Starting fzf in a popup window~
                                             *fzf-starting-fzf-in-a-popup-window*
 >
     " Required:
-    " - width [float]
-    " - height [float]
+    " - width [float range [0 ~ 1]]
+    " - height [float range [0 ~ 1]]
     "
     " Optional:
+    " - xoffset [float default 0.5 range [0 ~ 1]]
+    " - yoffset [float default 0.5 range [0 ~ 1]]
     " - highlight [string default 'Comment']: Highlight group for border
     " - rounded [boolean default v:true]: Use rounded border
     let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }

plugin/fzf.vim

@@ -843,10 +843,16 @@ endif
 
 function! s:popup(opts) abort
   " Size and position
-  let width = float2nr(&columns * a:opts.width)
-  let height = float2nr(&lines * a:opts.height)
-  let row = float2nr((&lines - height) / 2)
-  let col = float2nr((&columns - width) / 2)
+  let width = min([max([0, float2nr(&columns * a:opts.width)]), &columns])
+  let height = min([max([0, float2nr(&lines * a:opts.height)]), &lines - has('nvim')])
+  let row = float2nr(get(a:opts, 'yoffset', 0.5) * (&lines - height))
+  let col = float2nr(get(a:opts, 'xoffset', 0.5) * (&columns - width))
+
+  " Managing the differences
+  let row = min([max([0, row]), &lines - has('nvim') - height])
+  let col = min([max([0, col]), &columns - width])
+  let row += !has('nvim')
+  let col += !has('nvim')
 
   " Border
   let edges = get(a:opts, 'rounded', 1) ? ['╭', '╮', '╰', '╯'] : ['┌', '┐', '└', '┘']