Always execute preview command if {q} is in the template

Even when {q} is empty. Because, why not?

While this can be seen as a breaking change, there is an easy workaround
to keep the old behavior.

    # This will show // even when the query is empty
    : | fzf --preview 'echo /{q}/'

    # But if you don't want it,
    : | fzf --preview '[ -n {q} ] || exit; echo /{q}/'

Close #2759

CHANGELOG.md

@@ -68,12 +68,22 @@ CHANGELOG
       # Can only type numbers
       fzf --bind 'change:transform-query(sed 's/[^0-9]//g' <<< {q})'
       ```
-- Improvements
     - `put` action can optionally take an argument string
       ```sh
       # a will put 'alpha' on the prompt, ctrl-b will put 'bravo'
       fzf --bind 'a:put+put(lpha),ctrl-b:put(bravo)'
       ```
+- Behavior changes
+    - fzf will always execute the preview command if the command template
+      contains `{q}` even when it's empty. If you prefer the old behavior,
+      you'll have to check if `{q}` is empty in your command.
+      ```sh
+      # This will show // even when the query is empty
+      : | fzf --preview 'echo /{q}/'
+
+      # But if you don't want it,
+      : | fzf --preview '[ -n {q} ] || exit; echo /{q}/'
+      ```
     - `double-click` will behave the same as `enter` unless otherwise specified,
       so you don't have to repeat the same action twice in `--bind` in most cases.
       ```sh

man/man1/fzf.1

@@ -536,7 +536,8 @@ e.g.
 Note that you can escape a placeholder pattern by prepending a backslash.
 
 Preview window will be updated even when there is no match for the current
-query if any of the placeholder expressions evaluates to a non-empty string.
+query if any of the placeholder expressions evaluates to a non-empty string
+or \fB{q}\fR is in the command template.
 
 Since 0.24.0, fzf can render partial preview content before the preview command
 completes. ANSI escape sequence for clearing the display (\fBCSI 2 J\fR) is

src/terminal.go

@@ -2096,7 +2096,7 @@ func (t *Terminal) currentItem() *Item {
 func (t *Terminal) buildPlusList(template string, forcePlus bool) (bool, []*Item) {
 	current := t.currentItem()
 	slot, plus, query := hasPreviewFlags(template)
-	if !(!slot || query && len(t.input) > 0 || (forcePlus || plus) && len(t.selected) > 0) {
+	if !(!slot || query || (forcePlus || plus) && len(t.selected) > 0) {
 		return current != nil, []*Item{current, current}
 	}
 

test/test_go.rb

@@ -1555,13 +1555,13 @@ class TestGoFZF < TestBase
   end
 
   def test_preview_q_no_match
-    tmux.send_keys %(: | #{FZF} --preview 'echo foo {q}'), :Enter
+    tmux.send_keys %(: | #{FZF} --preview 'echo foo {q} foo'), :Enter
     tmux.until { |lines| assert_equal 0, lines.match_count }
-    tmux.until { |lines| refute_includes lines[1], ' foo ' }
+    tmux.until { |lines| assert_includes lines[1], ' foo  foo' }
     tmux.send_keys 'bar'
-    tmux.until { |lines| assert_includes lines[1], ' foo bar ' }
+    tmux.until { |lines| assert_includes lines[1], ' foo bar foo' }
     tmux.send_keys 'C-u'
-    tmux.until { |lines| refute_includes lines[1], ' foo ' }
+    tmux.until { |lines| assert_includes lines[1], ' foo  foo' }
   end
 
   def test_preview_q_no_match_with_initial_query